Possible options:

hp100_port = card#,IO-base[,card#,IO-base] (for PCI)
hp100_port = IO-base[,IO-base...]          (for others)
(Does EISA-probing, if on EISA-slot;
 On ISA-bus probes all ports from 0x100 thru to 0x3E0
 in increments of 0x020)

Greg Buchanan got two J2790A PCI cards to work in a 486 Vectra using:

hp100 hp100_port=1,0xfc00,2,0xf800