Ah yes, good old PVX :)
You mention you had already played with the session helpers, but here is a handy link for reference on that subject.
http://firewallguru.blogspot.com/2008/03/sip-and-h323.html
I know that internally Polycom does not pass video traffic through the firewalls because NAT inherently breaks most H.323 and SIP traffic. They use their VBP (Video Border Proxy) which is essentially a H.323/SIP proxy that knows how to manipulate this type of traffic properly even when NAT is involved.
Trust me when I say it will likely be much easier to get some type of H.323 Proxy than to get this working via the firewall. After all, I used to run the firewalls and VBPs for Polycom ;)
A Real World Fortinet Guide
Configuration Examples & Frequently Asked Questions
http://firewallguru.blogspot.com