"How would sex reassignment surgery play out for an unbinary person?"
I had to think this one over for a bit. It's a difficult thing to answer, because, really, for the most part, right now it wouldn't. Very few surgeons are willing to do it for a 'gender variant' person, as we are sometimes called, and very few therapists are willing to give the required evaluation to get SRS in such cases, and it's really expensive, to boot! So, getting there at all may be out of reach for most people.
What you have to understand first of all, and what I think a lot of cissexual folks fail to realize, is that SRS is actually quite difficult to get in most places, regardless of your identity. This is beginning to change, but very slowly (and, it would seem, with much backsliding). However, as of now the Harry Benjamin Standards of Care, a code to which the great majority of therapists subscribe, is still making life intensely difficult for anyone and everyone who is looking for 'the surgery'. Patients must be using hormones for a year before surgery - this is an extreme issue for agendered people, who wish to rid themselves of masculine and feminine characteristics, not just switch one out for the other, or end up with both! Patients must also have lived as their intended sex for one year - again, this is extremely difficult for an agendered person, as there are no accepted standards for What Agender Looks Like. I have heard hundreds of horror stories of transfeminine people having to act like real-life Barbie dolls to get their surgery, transmasculine people having to go completely dudebro for theirs... and 'gender variant' people having to pretend to be one or the other just to be taken seriously! A year of living like that, in a way that is so abhorrent and contrary to most agender principles, may not be worth it for a surgery that might end up simply tagging you with a different gender signifier, rather than what you actually wanted or hoped to achieve (hint: the answer is 'none', or at the very least 'none of that').
SRS is very different depending on what your birth sex is, your body shape, your hormonal levels, etc. A FAAB unbinary person may not even need top surgery if xe has a flattish chest already... and some do, sexually, enjoy the sensations in their chest and/or nipples too much to want to risk losing it with breast removal, so they just go on binding instead. But, for many FAAB folks, top surgery is part of the package. And as with transmen, this will often mean having to get into a good of shape as possible, since the larger and fattier the breasts, the more likely that the resulting scars will be puckered or prominent, which could mean a greater risk of loss of sensation. Some people may opt to get their nipples removed (were i to have top surgery, i would do this) as well, and sometimes nipples must be removed because of damage incurred during surgery. When it comes to Down There, many unbinary people, whether they be FAAB, MAAB, IAAB or otherwise sexed, may be fine with just tucking/ignoring those little fleshy bits away. Others may yearn for smoothness; in that case i believe it is ever so slightly easier to obtain removal-without-replacement surgery for people with male genitalia than it is for people with female genitalia, if only because it is less 'invasive'. However, this sort of surgery is likely much easier to obtain from illegal or otherwise under-the-table services, which presents many issues of safety and healthy, both mental and physical, as well as potential for exorbitant pricing or lack of aftercare. i have heard of a few tattoo/piercing places doing male genitalia removal as a 'body modification', but i highly doubt it is anywhere near legal; regardless, such a method may be the safest way to obtain such a 'modification' under-the-table, as bodymod artists do tend to have quite a bit more experience with the purposeful and artful injuring of the human body, plus better knowledge of proper safety and aftercare, than, say, a back-alley butcher. But then there is also the problem of where to find these people; illegal services are generally not openly advertised, and, as illegal activity tends to beget more illegal activity, even the services which can be found may be extremely sketchy and/or dangerous.
Simply asking a doctor to have one's sexual organs removed is not as simple as you may think; for example, i recently spoke with mine about getting the 'tubes tied, and in doing so made a joke that i would rather just get a hysterectomy, if it weren't for the fact that my hormones are all over the place as it is and there'd be no telling what effect it would have on my anxiety, depression, etc. In response to the joke, xe laughed uncomfortably and said, in a very serious tone, "Well, I would never allow a hysterectomy for someone your age, anyway." Allow. i also got subjected to a long ramble about how i might 'someday want children'... having never experienced being MAAB, i can't say if this is how men and boys are treated by their doctors as well, but this sort of condescending 'we know what's best for you' attitude is something i've experienced a lot of as a FAAB person. It's easy to see how such patronizing treatment might turn othergendered folk away from the medical community as an option for SRS, sadly.
Until recently, I would have said that true, medically-approved-and-performed SRS for unbinary people was a fantasy. However, Andra, one of the admins over at What Is Gender? has indeed had SRS to give it nothing more than a clitoris and a urethra. This may not be every unbinary person's desired result - i know i, for example, would not want a clitoris - but it's still a great thing, and very encouraging to hear of. And with Dr Drescher and co. working on changing how the medical community looks at trans*folk in general, and unbinary folk in particular - well, who knows what SRS options might look like in five years, in ten years? For now, though, SRS can be very difficult to get - and unbinary SRS doubly so.