Forgive me if I've misinterpreted, but I don't think the biggest problem here is running 120 film through a 220 insert+back, or vice versa. The OP is proposing to buy an ETR (6x4.5) insert and run it through a SQ-A back made to handle 6x6 film. I'll admit to only having one of the systems (the 645) so I could be very wrong, but I would imagine that said mismatch is not going to align properly. Width-wise, maybe, but do they match top to bottom?