I, too was thinking perhaps you could fab up your own 'blimp' out of some type of box, like tupperware, lined with HD foam (which you can get at craft/fabric stores) and you could make a tube for the lens out of PVC and put foam on the outside of that.
The XTi/400D's shutter is already pretty darn quiet, (kind of a "ss-kzz") so it shouldn't be difficult to really muffle it further. Also, I think they make a wired shutter release for them so if you use one of those, you don't have to worry about pressing the shutter button through the box.
I have an Idea in my head, but execution is a different thing...
The 70-200 2.8L IS is an awesome lens. I'm totally in love with it.
With the battery grip, you shouldn't have to change batteries all night, and if you get like an 8-gig CF card, you should be fine all night as well.
As for the "silent mode" on DSLR's, as far as I know, only the MKIII's have this feature, and it's kind of a small '
click- ta' sound. It's actually called the "Soft Feature".
Go
HERE and they have sounds of a bunch of different Canon's.
Definitely keep us updated as to how this turns out.