This is an interesting problem and actually close to solvable at the IP level. I'm sure you are familiar with PPP LQM. Take out the PPP part and keep the LQM packet format and local storage. Keep one LQM struct per ARP entry on a bcast or nbma interface (such as an ATM NAP). Count packets used by each ARP entry and update SNMP and the LQM entry as you would in PPP LQM. Occasionally (once a second is fine) send an LQM packet summarizing what has been sent.
This is a very good idea. You might want to hand it over to your vendor before I hand it over to mine. :-)
Hey, now. Curtis has been saying this for at least a few years. Clearly people haven't been paying attention! (but don't let that stop you from knocking any particular vendor about it). --jhawk