MAME is a pain, it won't take joy-to-key remappers, it only listens to low level input. So whatever you map your sticks to will likely have no effect. You might be able to do it with a registry edit, but anything short probably won't work.
I'd be curious to learn more about Kevin's program, though.
On the other hand you should just be able to bind both those buttons to UI Cancel in MAME. As in, set UI Cancel and just push both the buttons you want.
It's not perfect, it's not the hold7,push8 combo but it'd be the next best thing.