If you want them to experience it as a human body, just add software that makes it feel like they're moving muscles that aren't really there. Also, if you can find the right part of the brain to mess with, make them incapable of noticing something's wrong with their body.
Besides, pneumatic motors are tiny. It's not that hard to add more. Granted, they'd have to carry around a compressed air tank or air compressor if they want to walk around a lot, but that's not going to be a problem if you just want to hug your grand-daughter.
I made a video last month, which when I mentioned in another thread someone said I should post as a top level discussion.
It's just a ten minute zero-budget thing I wrote in which a girl has a video conference with her dead and backed-up-then-uploaded grandfather. Intended as the first in a series, but later episodes will only get produced if donations come. Later episodes talk more about AI's failures and the political situation with unrest from the living demanding the dead shouldn't have their jobs etc.
Anyway, watch it here if you like, I'd be happy to hear what y'all think :)