Hi Richard,
Thanks for sharing your experiment and results!
Personally, I've never been convinced that HDR photography has much of a place in astrophotography, except for the very brightest of DSOs (M42 comes to mind). Interestingly enough, your M42, which theoretically would benefit the most, still seems to have a blown out core.
I think that your 400D has a pretty good well- and bit-depth, with a low native ISO starting at 100 and therefore an ability to capture a good dynamic range without requiring different exposure times to counter overexposure. That said, I'm getting more and more of an armchair photographer these days, so someone else may want to chime in on this...
