I got it this far right now, but I'm almost 100% positive that I'm calling
i386_set_ioperm wrong (is that even the equivalent of Linux's ioperm?)

The original code calls:
ioperm(port_addr, 16, 1);

The manual pages, being completely over my head, led me to the following:
u_long *port_array = { port_addr, 16, 1 };
i386_iopl(1);
i386_set_ioperm(port_array);

The argument to i386_set_ioperm was a complete guess :|

And it gets this far:
blackhole:/usr/home/jmcneill/3c5x9utils-1.1.orig# ./3c5x9 
A potential 3c5*9 has been found, but it appears to still be active.
Either shutdown the network, or use the '-f' flag.
blackhole:/usr/home/jmcneill/3c5x9utils-1.1.orig# ./3c5x9 -f
3c5*9 not found at 0x300, status 0000.
If there is a 3c5*9 card in the machine, explicitly set the I/O port address
  using '-p <ioaddr>

Is there an easier (and more importantly, the right) way to do this?

Jared