The lack of reply should tell you something ;)
Until recently, it was not a decent possibility.
Now it's usable, and doesn't look too bad actually, but it comes with limitations.
The basic setting for each game should be:
Code: Select all
; Insert here [romset name as specified in the MRA]
vga_scaler=1
video_mode=320,8,32,24,240,4,3,16,6048
vscale_mode=0
The last setting is very important: it ensures the game will fill the screen and be somewhat readable. Now, the problem is that this way, it is not properly scaled anymore, which will introduce very visible graphical glitches.
The core needs to have the basic aspect ratio settings from the Main core implemented. This means something like Gemini Wing will work relatively. Some other cores will display with worse aspect ratio. Most cores (such as the ever popular CPS1 core) won't work properly currently in such a Frankenstein setting.
The use of user-set custom aspect ratio introduced recently should help with the aspect ratio eventually, but, again, each core will have to be updated to incorporate this, and most of the arcade cores are very old — in MiSTer age, that is ;)
I didn't run many tests, but in my experience the only cores currently having the necessary basic requirements to make it working were Nullobject's Tecmo (Gemini Wing) and Jotego's beta Contra core.
The relatively decent results (all things considered) are mainly because of the low resolutions of those games. I'm pretty sure Cave shooters will be nearly unreadable (as they were, when rotated, in their Sega Saturn ports), and I doubt the CPS1 ones might be either.
It's a curiosity more than anything, and offers very limited interests versus simply emulating the games on anything — and no interests versus using a modern screen.