Well, theoretically you could disconnect Antic completely from GTIA and drive it so long as you sent the right commands at the right time and had something fast enough to change what it was displaying.
Some observations though:
- with the "scanline 240 bug", the DMA setting will affect the "curtain", ie turn it off completely or alter it's width. You can change colour of PF1 at will and it will be relected there too.
- Maybe Antic has a bug similar to the earlier post where some GTIAs can be fooled into shifting GR. 9 pixels. If you display hires on the last line, it "forgets" to do some of it's housekeeping.
Another thing - you don't even need hires on scanline 240 to activate the bug. All you need to do is disable DMA further up the screen during a hires mode line.
There's no register to send all the commands to GTIA (in software); there's the "bus-load" method for P/M graphics but not for AN2..AN0. In modes like Graphics 9, AN0..AN1 are mostly just the data DMAd but with ANTIC off, what register do you use to set AN2..AN0?
I guess you need to generate a scanline or part of it so that the delay of GTIA/ANTIC is less than a CPU cycle. I read that the CTIA was HALF color clock delayed from GTIA. So if you can "throw off" the timing of the GTIA, it should cause a shift like Graphics 10 already does.