I own and have installed Shady Lewd Kart, Marble Maid, and Project Cappuccino, all through Itch (rather than Steam). While playing SLK, I went to Lock N Lewd and was able to redeem MM, but I can't seem to redeem PC. In fact, the latter is still shown with a lock on it. I skimmed the patch notes for the December update and it sounds like it *should* be compatible, so I'm at a loss as to why it's not being recognized.

Immediate update: Apparently, I just had to launch Project Cappuccino again. It seems I hadn't touched it since the second-to-last update, so maybe the game wasn't "reinitialized" yet? In any case, without even closing SLK, just launching PC and reloading the Lock N Lewd page unlocked the cross-game feature. I'm leaving this thread for posterity, since I wasn't able to find anything on the topic myself when I was looking into it.