Hey Raz,
I might know of a solution for you. I haven't tested it, but there is some source code out there that supposedly adds this functionality as a regular IP_masq module. It's reportedly tested with Netmeeting and works great. I haven't had a chance to try it yet, but I did download it, and it compiled and installed fine on my RH7.0 box. Try this:
http://www.coritel.it/coritel/ip/sof.../nat2/nat2.htm
If that doesn't work, I can just email you source for the module.
Oh, I believe that does only allow for outbound calls to a regular h.323 user. But, I think you could just port forward the 'calling' port to another user to directly contact them.
Thanks,
Jon