So I managed to grab some boards for very cheap. They were listed on ebay as basically unfixable, but along with 11 untested gamecard boards. So for £40, that seemed like a win.
In all of it there were 3 normal switch boards, one was clearly already used as a parts board and was missing all sorts of things. But the other two were less clear cut.
Of these board A had a number of things missing, but nothing major, just some passives near the common problem chips, and also the LCD driver and SD connector. But on putting back all the passives and reflowing the USB I was able to get into RCM and inject a payload. This seems fixable, so I have ordered an LCD driver and SD connector so I can get it working.
The other, board B, was in similar condition but missing M92 and had a damaged fuel gauge. I replaced the passives again, and got it in to RCM, but it is patched.
So this leaves me with the situation: I have 3 emmc boards that were not connected to their motherboards. I think the way forward for board A is to get a display and sd-card working, extract the keys and then try them on each emmc connected to a PC to see which one it works on.
But how can I work out which of the remaining emmcs goes with the other working board? Is there a way? Or do I just have to pick one at random and hope that its either correct or on the same version?