I have had little to no luck booting from anything but drives hooked up to the internal SCSI bus for unsupported hardware like you are describing.
I've installed OSX since the beta days (and even a Rhapsody release or two) on the 7300/7600/8500 series of machines. The installations that *always* worked used drives on the internal bus with formatted drives from the "Drive Setup" utility from Apple or its precursor. (I can't for the life of me remember what it used to be called -- my how quickly one forgets!)
With that in mind, Jaguar is a new release and with the utility application from Ryan Rempel called "XPostFacto" many people have easily installed and gotten OSX running on this older, still useable hardware.
The catch for you would be installing and getting the system to boot from a third party drive connected to a third party card with a formatting utility other than Apple's. Some people have posted success with other formatting utilities, and I have used FWB Hard Disk Toolkit for formatting additional drives that didn't include the actual system and boot drive.
You should be able to install the system on an internal SCSI drive and have your extra drives on the ata card and use them as secondary data drives. If you know enough about the command line you could even edit and have your 'Users' directory moved onto one of these drives. It seems the booting portions of the code are what cause the trouble.
Good luck and post your results if you decide to go through with this. Right now, the limitations and cost of SCSI drives that are sizable for use with OSX are hard to come by affordably. I'd love to be able to buy another inexpensive IDE drive and card to have more space on these older machines of mine.