Define “transhuman”.
Nothing prevents a Solar from replacing an arm with a Prosthetic of Clockwork Elegance or injecting themselves with a Chaomorphic Symbiote or undergoing a few treatments with Science of Mutation. In that respect, all Exalts are “transhuman friendly” in that they can certainly have transhuman upgrades just like normal humans.
However, Solars are not going to develop a Charm that functions specifically on their mechanical arm in a way that it wouldn’t on a normal arm. They’ll never develop a Charm that grants them extra attacks based on how many tentacles their symbiote has grown, and so on. Neither will most Exalts. Infernals can sort of do it, but only within a very narrow theme based off their patron Yozi (or Devil Tiger incarnation) and Abyssals can do it within the themes of being a horrible undead monster.
But yeah, if you want to develop Charms that specifically key off having a non-human body plan – any nonhuman body plan – you want Lunars. An Infernal of Kimberry may get better “horrible acid tendrils” Charms than a Lunar would, but a Lunar can get both “horrible acid tendril” Charms and “ears like a fox” Charms and “I turn into a glacier” Charms and so on.