I haven't seen a commercial app to do exactly that. You could probably call the lobby ambassador functions from WCS from the phone (via browser or wcs api).
If it were me and I was looking for something easy to do, I would write a quick front end on the phone to an external server that just adds it to your guest WLCs via snmp (same way WCS would do it).
If you dig around 188.8.131.52.4.1.14184.108.40.206.1.24 in the mib, you can set users this way. I did a quick capture of WCS adding a user, it looks like it would be pretty easy to re-create with a script that you could call.