No idea can be deemed stupid until you try it and fail. Then we can say, "That was stupid".
Without going into a long rant here, I'd say you are facing a few unknowns. There are many models of hang on the back filters, whose to say none could be adapted to be an overflow. Off the top of my head I can't think of how it would be feasible. You could spend more money on buying filters and trying to adapt them than just buying a good hang on overflow. Not to mention the frustration and floods you're likely to experience.
As far as a siphon loss with a power failure, there must be a design out there that takes this into consideration and has solved the problem. Some one with experience in hang-on overflows will chime in here. I haven't used one since I went to Acrylic aquariums way back in the early '70s. So, I'm not familiar with the newer designs of hang-on overflows.