Transhumanist is a label. Like all labels, it is an attempt to categorize a set by creating subsets and giving those subsets names. This is generally a good thing, so long as this isn't an attempt to pigeonhole. Pigeonholes are those physical dividers found in old desks, and thus a metaphor for exclusive categorization - you can't put a paper in more than one pigeonhole. The problem with thinking categorically happens when the categories become pigeonholes. Once this happens, the only way to sort within that pigeonhole is to subdivide it. By using non-exclusive set definitions, one can look for much more interesting categories, such as the intersection of the set of womwn who are also of the set red-headed, and also of the set green eyed; or the intersection of the sets novels, science fiction, and far future.
So - transhumanism is a philosphy which advocates improving the lot of humanity by changing humans on an individual level. Of course on an individual level what constitutes improvement is a question of personal taste. One consequence of this is a broadening of the meaning of the label "human" into what is not necessarily recognizably human any more. The problem with most games that are considered transhumanist is that the center is moved off of the recognizably human, and onto something which is difficult to conceptualize, and hard to relate to. This I believe is a mistake with games, and comes from over-zealous interest of those writing and designing these games in the seductive individual variance of transhumanism. There is no safe space, no anchor to our world which people can use to explore from.
In StarCluster 3, I have throughout insisted that the center of the game be on recognizably human characters. They think of themselves as human, they act in a very human way out of human impulses. This is not to say they are the *same* as we are - they are on average smarter, more fit, more resistant to disease, and far more long-lived - but they are the baseline, and if one walked down mainstreet at noon, so-one would bat an eyelash.
What accounts for this? I mean this is in the far future in an unbelieveably advanced technology. They can be whatever they want to be! How is this justified?
Mankind is a social animal. Human society has persistence and tradition, and it also has mating rituals. It can be thought of as a kind of social gravity - the more human you appear, the better your chances of mating successfully. The less human you appear, the further out from the center you are. Thus the fringes of humanity are home to clusters of strange quasi-human mini-cultures and drifting individuals of extreme variation, while in the center is the great globe of those indistinguishable from humans.
What about putting conciousness in a machine body, or other transhumanist concepts like serial-body immortality? This can be done, but changes of body incur losses. Nothing comes without a price. The further from your base state you move, the more you lose in skills and memories, and the best equipment and highest skilled practicioners can only mitigate that loss. Some find that cost acceptable, but most don't.
So if you want a character who is a human centaur, there are several ways to do it, but in being a human centaur, you are cast adrift from the center and slowly migrate out to the fringe. Your choice.
So, the center is fixed, but the fringes are nebulous, and the question of acceptablilty is a personal choice. If one player chooses some radical variation for her character, that is fine, but the center is there - easily comprehended, somewhat idealized, but still recognizably human. Most will choose that path, based on my experience in play for many years with StarCluster 2. Is it transhumanist? That's your call.