It could be that OS X doesn't have a driver (kext) for the card. Can drives attached to the non-booting SCSI PCI card be mounted and accessed in OS X if you boot off of a supported controller in the same machine?

XPostFacto's boot support using a "kickstart" drive only works if the controller has an OS X driver. In the case of Firewire its the generic OHCI driver. Its mostly to get around the lack of ROM booting support for Firewire/USB in pre-New World machines, not the lack of drivers. The kickstart drive just loads the kernel and enough drivers to get the system running and transfers control to the unsupported boot drive afterwards.