Well, I looked over the board a lot more thoroughly today. Checked the caps, QFPs, DIP ICs, all the legs underneath the board etc... I didn't find a whole lot wrong, cleaned the board a bit more, didn't find any bent pins in the riser card plugs. I did find one odd thing though, on one of the big black connectors on the left side of the board (relative to where the JAMMA edge is), I found one pin was bent far enough to touch two others, so I bent it slowly back into position. I powered the board up with no riser card, LED came on and my PSU wasn't grounding out. I then plugged the riser card in and re-attached the metal cartridge guide. Again, successfully turned on. Thanks for all the help so far guys!
Now onto level 2 I guess heh, it's stuck in POST. I don't know what exactly the board is doing, must be some corroded traces (or bad chips). The signal is probably unstable, since a CRT won't pick the video signal up. It works with a Plasma though, so here's some pictures.
This displays with no cartridge inserted:
With a game plugged in, this happens.
Please keep in mind that with both pictures, the video signal flashes different garbage partially. The images above aren't static like the pictures may suggest.
I notice the board sort of looks corroded in a couple places, like one side of the big ASIC in the middle, the traces on one side of a little surface mount DIP chip and a couple other parts. I get the impression maybe this board isn't worth fixing, but hey I'm not the doctor here, perhaps it's not as bad as I think. I can take pictures of the PCB if you guys want.
EDIT: Pics of suspicious parts of the board added (and one for just the general sound area...) I can provide larger versions of these images if need be.
On one other note, I put the cart I tested with the MV-1 into a working MV-1AX and it go to a black screen after post. I wiped the cart edges with a shirt and black dirt/dust came off, so me thinks the MV-1 is also suffering from a dirty cartridge slot as well.