Back to "video modes show up fine in Picasso96 setup test-mode, but not in Prefs:ScreenMode."
You ought to have the Monitors ready by using LoadMonDrvs BEFORE calling/running the "IPrefs" program in S:S-S, otherwise they won't be "seen" by the Prefs:ScreenMode when you use it. Yeah you could do it by hand, but LoadMonDrvs does it simply compared to the older S:S-S code of ~ list Devs:Monitors T:M blah blah execute blah blah.
I learned this doing multiple installs for my mediator board. Either method will work, but some folks with only an NTSC or PAL, delete the code and just let the OS use it's default setting. Trust me, the new monitor settings provided by the P96 program have to be initialized by a "reboot, LoadMonDrvs (or equivalent), and then IPrefs" before being seen in Prefs:ScreenMode. It would be helpful for you to get someone else's explantation than mine, well someone who read the whole thread before commenting, as I can be confusing in the way I talk about a problem.