Actually, it is exactly as dull as it seems, but that's a complaint for a different thread.
As far as this thread goes, it would make absolutely no sense not to allow machinist to install cybernetic limbs, such as legs and arms. Synthetic organs, on the other hand, should be installed by a surgeon, given it requires the removal of said organ in the first place, but maintenance/repair of them should fall to the machinist first and foremost. In the absence of one, sure, get a tube of nanopaste and go wild, but the machinist job is already a role that can do very little without two other roles supporting it, a proper science role and a miner, and that's assuming either does their job properly, and on top of that a good portion of their job is made redundant by a tube of nanopaste. Removing even more from them, IE installation of cybernetic limbs, not that such is done with any regularity anyway, makes no sense.