I just got my LED pars in this week and will be creating T bars after the holidays, so thanks for the inspiration. I am also planning on making custom cabling (I hate the look of "extra" cabling too). I LOVE the expandable sleeving idea and will be stealing this idea directly from you.
Some questions: What's the AC inlet on those fixtures? I've never seen the back of those, it looks quite cumbersome with the "pack" on the back. How did you handle the AC coupling? Are the fixtures paired and spliced together (wire nuts, solder, etc?) If so, I'm assuming that's why you have two Edison tails in your cable harness... Why not just tie all 4 together at the T bar? I am going to have to decide on my AC configuration. On one hand I want the tails to be as short as possible and not necessarily have to use a quad box or similar. Though, on the other hand, there will probably be times where I need to disassemble the T bar to use the LED's as uplighting or for individual placement. I wish I could just buy more and not have to worry about it, but we all know how that goes...
Anyway, nice job, it looks very clean. May all your brides be impressed!