Thanks for the tip about the mapper; that get me a little closer. For best results, it seems like DOSBox-X (and maybe other forks) would solve my problem by providing CONF options, but I'd still love to see an easy-to-use implementation in DOSBox itself.
I've got two Spinnaker Software games, Alphabet Circus and Alphabet Zoo, that look best in "late CGA."
There are actually two issues:
1) "Auto" doesn't trigger composite mode for them. Presumably this is because their specific resolution/color combo can't be auto-detected, as discussed earlier on this page. I tried various combinations of this thread's "register ticking" programs, hoping I could put one in a batch file to execute right before the games, but to no avail. Would something like that theoretically be a solution?
2) Not only must the user (my 3-year-old daughter in this case) press F12 to enable composite, but she must also press some other key or key combo to get into "late composite" for it to look good. This just isn't going to happen. And as I ponder the learning curve for these settings, it seems like the early/late emulation is distinct from all the other controls that correspond to knobs that would have been on composite monitors at the time these games were new. This is why I like the idea of overloading F12, so that someone new to composite CGA could learn "Press F12 until you like how it looks," and could eventually get into the F11 stuff if they want to adjust the (virtual) monitor.
Thanks for any further tips.