Hi!
I had a problem like yours (with another brand pci ide-raid controller), you have details in
this thread.
Fortunately we had the sources of the driver (ready to work with 2.6 kernels). I compiled it into the kernel (not like a module). And I left a small disk (800 mb) whith /boot partition (and kernel image) and another partition with a minimal debian distribution for rescue purposes. And all the rest of the stuff into the disk attached to the raid controller.
All working OK.
Well, if you have a megaide.o driver, you can copy the file to a floppy and, in the beginning of the debian installation, you can load this driver to have support of the drives attached to the raid controller. Debian insert this megaide.o driver into his default initrd, so you have support of your controller from then.
Hope to help you.