First up thank you for making this game. It looks fantastic. I have got a copy of PA and I just can't get into the whole theme. There is after all only so much a prisoner is able to do!
However like the original OP said, I would be very careful about potential litigation (I am not a lawyer) of using the same art style. Seems to me to be far too uncanny and therefore its likely it will be seen as a blatant attempt to hijack on the back of PA's success. I don't think the theme is enough to justify the change, as it could just as likely be a free to play (developer sanctioned) mod. Might want to check with a solicitor if you haven't done so already?
I personally would prefer if you went for a more gritty realism art style
I'm thinking that even something akin to Cannon Fodder, Sensible Soccer top down looks would be great. You can definitely get in enough characterization using just hair colour, skin colour, build, hair length and clothing without needing facial features.