To unlock HoTD2, you just have to beat HoTD3.
No Virtua Cop available (though 3 runs on Sega's Xbox based system in the arcades, and would make a great port, but I wouldn't count on it

)
Other emulated games, are Point Blank in MAME, though it doesn't have any sound. Zero Point 1&2 both work excellently, and do have sound though (These games are pretty much identical to Point Blank).
I don't what all was available on SNES, but the emu is compatible with light guns.
NES had Duck Hunt, as well as several other fun shooters. There is a platformer game where you use the gun advance through the level. It is some great fun, but for some reason my mind just went completely blank trying to remember the name.
The Sega Master System emu also has light gun support, but again I don't know what light gun games were available.