> On the PC architecture, ISA bus devices only decode the bottom
> 10 bits of the IO address space.

Sorry, this simply is not true.

For a counter example see the "USR ISDN Sportster TA intern" cards, driven by
sys/dev/isa/isic_isa_usr_sti.c. I have such a card and it definitively works.
It uses the following offsetts

#define USR_HSCXA_OFF   0x0000
#define USR_HSCXB_OFF   0x4000
#define USR_INTL_OFF    0x8000
#define USR_ISAC_OFF    0xc000

from it's base IO port for different chips (or parts of chips) on the card.

Martin