Ok..did some digging...the problem is the "Bridge Chip" used to convert the 7800PCIE chip to work with AGP is RIGHT smack dab in line with the mount holes!
I almost gave up, or was going to tel you you might try and mod a standard watervlock, when i found this thread:
Gainward Bliss 7800GS+ AGP Waterblock? - HEXUS.community discussion forums
The "Aquagrafx" block is specificaly designed for a 7800GS!
This thread mentions it too.
Waterblock for the 7800GS (AGP) - Futuremark Forums
It's a German company, so your on your own as to how to get it imported here, I do not know of any retailers that stock them....but here is a link to the company, the block is towards the bottom.
aqua-computer - GPU-coolers
Sharka Computers is the only place they list as carrying there products in the US, but I did not see that particular model offered, maybe if you call them and ask nicely, they can hook you up.
Water Blocks - VGA