> I am building a custom cabinet. I'm thinking of buying a 16:9 monitor, rotating it 90
> degrees clockwise, and covering up the bottom so that what is viewable to the player
> looks like a 4:3 monitor.
> In order for this to work, I'd need to have MAME rotate its game display—I know it
> can do that—but also, draw the upper left corner of the game display in the lower
> left corner of the monitor. In other words: both rotate and reposition MAME's game
> Is this possible in any MAME builds out there?
Sure, it should be relatively easy if you come up with a custom .lay file for it. However, I'm a bit light on the details at the moment. I would suggest looking at the existing .lay files supplied with the stock MAME build to glean ideas.
Can I ask, though, why you're intent on masking off part of the display? On this very forum you can find countless high-resolution scans and "digital remasters" of bezel artwork that was traditionally always surrounding the monitors of various arcade games. Why not look towards filling in the otherwise-blank areas of the monitor with that, rather than wasting a significant amount of your visible area by masking it off?