I also tried creating a 0/0 VS listening on 67, first on IP forwarding mode and then L2 (not expecting it to do any good), in the hopes that TMM would recognize 255.255.255.255:67 as something to react to, and it still didn't show up in dhcrelay, even though I could see the packets in tcpdump the whole time.
So I guess what it goes back to is whether or not there's some other place that's akin to "b db *" that has a setting to handle broadcast packets, too, instead of just silently ignoring them or whatever its current behavior is.
... I guess I should add that I also have a 0/0 IP forwarding VS that load balances all my outbound (SNAT'ed) traffic through two routers. I doubt that would affect this, but it might be relevant info.