For games, like SM64, which are in series, the series page has an option right at the bottom of the sidebar called 'Game type'. If you click on the 'not' button, it'll change to 'has' and shows any ROM hacks and other game types (fangames, flash games, betas) in the series. It's more indirect than finding each ROM hack on the right game page, but I guess it got moved around in the large update recently.

Yeah, that method sort of works for most things for now. I'd still like a way to get all games with a given game as the parent game.