1. I would build a cabinet and have it run off of a Dreamcast with house of the dead 2
2. I would have this homebrew / mod disc loaded in first, and then swapped with the "House of the dead 2" disc.
3. The homebrew would scan the ram for the text "Press start to play" and change it to "Credits 0/2" Then I would have a coin slot rigged up so when a coin is inserted it connects the X button to gnd, and the homebrew would change the text to "Credits 1/2" when X is pressed. The idea is that the X button isn't on the light gun, so there's no way to get free credits. Canceling out the start button actually starting the game seems like it would be a big hurdle, but, its just an idea. Now to get coding