I don't think it's really possible, but I haven't had the occasion to try with my own 6309. I know for sure that specific irq's can be reserved for isa cards, sec 3.6 of the manual. I also know that disabling ACPI by selecting "compliance with O/S" as NO in sec 3.5 forces a "normal" irq distribution. I had to do it that way because my vidcard is not acpi compliant. Wouldn't wake up, except from a reboot. Control panel reguired some sorting out, but everything works just fine with APM.
Why do you want to do this? Do you have a card that demands a particular IRQ or set of IRQ's?