It can be done. Go to Devs:Monitors, and single-click your graphics card driver icon, then press Amiga+I to bring up the Icon information window. Change the "DISPLAYCHAIN" tooltype to no, and reboot. Now, when something uses the Amiga native modes, the Picasso96 output won't go blank. You'll now be able to see AGA modes in your TV card window.
You can even set up an ARexx script for each game to load SuperTV if its not already running, set it to full screen, run your game, then quit SuperTV when you quit the game!
Something like: (and i can't test this because i'm not at work)
if ~show( 'P', 'SUPERTV' ) then do
address command "run <>NIL: dh1:SuperTV/SuperTV"
end
address supertv
tv_setchannel 6 /* or whatever channel */
tv_fullscreen on
address command "MyGame"
address supertv
tv_quit