Onto initial observations of the Episode III Trial Edition...
For some reason the game starts (and rather abruptly stops) playing the opening theme of Episode I on the screen asking you whether you want to display in Progressive Scan mode. Not sure why they'd do that, but whatever...
The cinematic that you get when you sit on the title screen for too long appears to be the same one that was used in the final version of the game (at least as far as I can remember), complete with horrible English grammar. However, the opening cinematic when starting a new game is... well, lazy. Not quite as lazy as the one on the Dreamcast NTE, but pretty close. I don't remember it being that way in the final version, but maybe I'm just remembering incorrectly.
Just like with the Episode I & II trial, there's no option to change the language from Japanese, which makes things a bit more difficult for me to mess around with, but whatever.
There does appear to be a regular offline mode, although I can't read anything to see if there's interesting functionality missing. At some point I probably should play through the final version of Episode III to try to figure out what to expect.
As for online mode, the Episode III trial does use the normal Gamecube version encryption scheme, and works fine with Devolution. It connects to pso-ep3t.sonic.isao.net on port 9002 (so, I added the hostname to the Sylverant DNS and the port to the login server in r720). The game can connect just fine to Sylverant at this point, and you can get to the lobby on a ship. However, for some reason, there's no background music once you do. Not sure why that would be as of yet. Starting a team seems to make the game crash, and thus disconnect at this point. So, it's only slightly less functional on Sylverant than the final version of Episode III.
So, much like the final release of Episode III, there's a lot of work that would still need to be done, but it does at least connect at this point with minimal fuss.