Is there any way to set a specific aspect ratio?

[SirPrimalform]

I'm setting this up and I'm having trouble finding out how to set the aspect ratio for systems that aren't 4:3 and don't have square pixels.
I've found the "maintain aspect ratio" toggle, but this is only good for systems that are 4:3 (set it to off) or have square pixels (set it to on).

Some systems like the Master System don't fit neatly into either category as although 256x192 is 4:3 on paper, the system was drawing 192 lines in 240p mode so they were letterboxed on the TV by 24 blank lines above and below the game picture.

So I'm wondering how I can specify the aspect ratio in more detail for the systems that need it? I know how to do this on RA on the PC, just not sure how to do it on this version.
Thanks!