I was trying the supl proxy on my server and pointing my N900 to it. However results were inconclusive, I wasn't sure if my N900 was even happy with my supl server's certificates, making it tough to see what the real problem was...
I definitely did see some wireshark activity on port 7275 however, but didn't pursue it further after getting T-Mobile's server working... Granted a public server would be better as then it would also work on wifi...