I can think of a few ways:
a) change the sequence of loading what modules when in the boot process. That requires a rebuild of the initrd file, if you use one. If you don't, you'll need to rebuild the kernel and edit it's makefile somewhere...
b) swap the IDE controllers in Grub. I've read about it somewhere, can't remember where though. (errr, I can after some thought: it was in a Dutch forum, so unless you can read Dutch it won't be of any use I guess)
c) tell Grub to boot from the other controller. Don't know how, I presume somebody documented it somewhere.
Just what I could come up with in this short time. I didn't do research on this, but I know you know how to use Google