There are some older routers that will glitch if you don't have a *valid* mac in there, so try with a 11:22:33:44:55:66 address (or all zeroes etc.). On the whole though, if you can't sniff a client, or you can't brute force a MAC you're SOL. Another thought occurs - an old pentest revealed that if you could detect a wired MAC it would (in some cases) let you inject frames on the wireless side. I found this out by accident, misread a capture file.
Basically, just try things out and see what happens. You never know.