It lists native res and other stuff, but not target res of their screens.
EDIT:
https://github.com/mamedev/mame/blob/master/src/emu/rendlay.h
EDIT:
I examined FBA-rr code, and whenever it does aspect ratio correction via user option, it uses 4:3 or 3:4, depending on the game orientation. So this is now known to match the regular CRT aspect ratio.
MAME, among its systems, emulates arcades such as pinball machines and LCD based ones that can have arbitrary aspect ratio and it won't change it. But for all the rest, there's a bunch of possible layouts that determine the resulting aspect ratio. Again, 4:3/3:4 dominates here, even for multi-screen ones, but when I have time I'll make a list of games that need no aspect ratio correction, for future reference.
Bottomline: Whenever in doubt, apply regular CRT aspect ratio. If it's a multi-screen system, each screen usually is also a regular CRT.
To be continued...
http://tasvideos.org/forum/viewtopic.php?p=451786#451786