Not even I have a UMA option there on my T-Mobile 8320.
I took a look through my list of service books too, I can't seem to find anything related to UMA in there. Looking back at astro's screen shots, I notice that the WiFi diagnostics is missing the UMA there too. I should also mention that I'm not using a T-Mobile OS, so I don't think the functionality is in a specific provider OS. As far as I know, device specific firmware is really just a thin hypervisor type layer that provides a Java Virtual Machine, so I doubt UMA specificity is in there.
The hardware is, for all intents, and purposes identical. The wireless routers that T-Mobile sells have the WMM power saving modes enabled, which the phone can use to (drastically) decrease the power utilization while in WiFi, but the phone side wireless hardware should be identical.
If UMA behavior is not defined in service books, operating system version, the only two options are certs (which you installed) or the WiFi application in the OS might check for vendor ID.
I have one more idea as a long shot if some of my previous assumptions prove false:
Can you use javaloader to list the installed modules on your device and see if net_rim_cldc_gan_diagnostics and net_rim_cldc_impl_gan are present on your device? Generic Access Network/Unlicensed Mobile Access are the same tech.
In the event they are not, and if you're feeling particularly adventurous, you can try installing the modules with javaloader (after backing up!). It could be that the OS installation by the desktop manager checks vendor ID and excludes packages as needed, but javaloader will let you install it.
The more I think about it, the more likely I think that those modules might be missing from your device.