Is there any way to manipulate a malformed contact header in a REGISTER packet *before* we run it through proxy_auth, etc.?

I've got a couple of phones behind a Cisco router, which insists on lopping off part of my Contact header. Basically, my inbound REGISTER packets shows the contact header as Contact: sip:foo.bar@192.168.1.1:5060;transport=udp;line=8hzddhav;q= (note the trailing 'q=')

What i'd like to do is append a "1.0" to the header, and *then* run it through the proxy_auth, etc., etc.

If I do a subst(), it doesnt seem to help.

Any ideas?