PCI graphics cards with PassThru

[CLS2086]

One the Amiga Side, you can get the signal at the entrance of the VP101-3BA (or ADV191KP30 depending of your A1200 mobo...) :
http://assets.zarlink.com/DS/zarlink_VP101_JAN_01.pdf
but i'm sure that you'll need at least a buffer system like on commodore VGA adapter.
Good Luck.