I don't think ARP is the right solution for this problem.  What you have
is NBMA (non-broadcast multiple-access) medium and arp just isn't
appropriate.

All you really need to do is modify your if_output routine for the device
to grad the address out of the route.  If there is no route (or address in
the route), return EHOSTUNREACH.

To populate your cache, simply do:

route add 10.1.2.3 rn0:1:2:3:4:5:6:7:8 -interface

(the driver can convert the 1 to 0x31 etc.)

Matt Thomas               Internet:   matt@3am-software.com
3am Software Foundry      WWW URL:    http://www.3am-software.com/bio/matt.html
Westford, MA              Disclaimer: I disavow all knowledge of this message