I just checked in a small tool to HEAD in
/usr/src/tools/tools/ether_reflect which uses pcap and bpf to reflect
ethernet packets just about the driver layer without involving the
protocol stacks. This is useful for people doing low level testing of
drivers and switches. If you happen to be lucky enough to have an
ethernet packet generator (ixia et al) this will do what you want in
terms of reflecting the packets back.