Been trying to do the same thing tonight. Got OpenVPN working with PAM and Google Auth on my Raspberry Pi, so wanted to do the same on OpenWRT.
It looks like there isn't a ready-made PAM plugin for OpenVPN for the latest OpenWRT, which will scupper any attempts to use OpenVPN with Google Authenticator on OpenWRT. At least, I can't see an OpenVPN PAM plugin in the make menu for OpenWRT (Chaos Calmer).
I've never attempted to cross-compile from source, but when I get some spare time I may give it a go. Who knows? I might get lucky and somehow make it work.