Wonder about it no more....
That door leads to the "HK FACTORY", and for laughs and giggles I played through the side quest last night! NO, you can't get there through the door. It was originaly planned that there would be a side quest with HK, called the "HK factory" (it comes up in conversation quite early on with HK, but oddly led to nowhere).
As it turns out the DEVs did quite a bit of work on the "HK FACTORY", it is however only accessable currently by downloading a modified save game file, that can be copied to the xbox harddrive via an "ACTION REPLAY", or similar device (thanks STORMSINGER!)
http://home.alltel.net/jls31/
There's the location of the save game file you need. Aparently there is more to this game than meets the casual oberver. Enjoy.
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How much would you say is complete on it?
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It's approximately 3 maps long (there may be more to it, I haven't had time to play through it more), and numerous encounters to be had. Overall it is certinly larger than the original TELOS military base by quite a bit, and larger in number of total rooms than the spacestation ('I'll try to get some pics of the maps involved this weekend as well). Even with the little bit that I played through, it seemed oddly close to completion (sometimes a conversation will lead to a cutscene, where there will be text requesting a dev to create the scene, however the scene will play as if it was done).
Unfortunately it can only be played solo with HK (and he is powered up beyond any challenge!), but if you're an HK fan, there's a lot of story and dialog (humor) that you'll not want to miss. A wild guess might be an hour or so of playtime exploring, and going thru encounters there.
I quite inadvertently completed to quest (shut down production, and the facility reactor), and took the lift back to the TELOS military base, only to be given a formerly unsen movie of a Sith/republic war going on above... After that it locks up in loading.
It seems very near completion, they obviously put a lot of time into it, shame it got cut.
It makes me wonder if there are other "missions" on the disc that didn't make it to the "final" version.