so wire the mame buttons backwards using the gnd as positive NO as gnd? is the pinball NC connected to anything?
For some reason, people have it in their heads that the Common (COM) tab is
always and only ground. (I still need to edit a few pages of the wiki that contain this myth.)
From an electrical standpoint, that is the same as saying that a piece of wire has a positive and negative end --
total BS.

Most arcade wiring does use the COM tab to connect the ground wire, which ensures that the NC terminal will only have ground on it in case a loose wire happens to come in contact with it, but it's not the only way to connect things.
how about wiring 2 buttons in 1 input for the flippers?
The NC from both MAME buttons goes to the pinball button input using a Y-connector like this.

With the "backward" wiring, when you to press one MAME button, the COM lever is moved from NC (connected to flipper) to NO (ground) which keeps you from grounding both MAME buttons at the same time -- same trick that's used for KADE Extended Mode.
When you press "Up", only the up input is connected to ground.
When you press "Down", only the down input is connected to ground.
When you press "P1 U+D", both the up and down inputs are connected to ground.
Wire the MAME buttons like "Up" and "Down", and the Upper Flipper/Magnasave like "P1 U+D"

Scott