Basically an open question, with some thoughts jumbled in my head. I don't want to think of this as just mashing together cyberpunk and Planescape (though I do think the dystopia scent does touch Sigil at the least).
I mean we could have steam punk cyborgs and Daelkyr-esque symbionts. But in some sense almost all the factions & sects are trying to make the basic bodily forms found in the Wheel into something more. And overall the Wheel is a post-scarcity society, depending on how you look at it. (Doesn't mean everyone gets to share in those infinite resources.)
So I honestly don't know - is Planescape already a transhumanist setting, only a fantastic one instead of a science-fiction backdrop for adventures? Or does the presence of fantastical themes negate the transhumanist concept?