How about some variation of tethered capture? If you're willing to hook up a laptop, you should be able to control exposure over a series of shots w/o disturbing the camera.

You may also be interested in an Android-based DSLR controller that hit the market last year. The 40D is one of the gray-area cameras that's supported, but not with full functionality (its OS doesn't expose as much functionality as the newer cameras, I guess). This app is still in "beta" testing, so although it does list an HDR mode right now, discussions in the support thread on XDA Developers indicate that there are still some teething problems. Still, the concept seems to have a lot of promise once it matures a bit.

Finally, this would be a bit of a long shot, but there's a project / product called CameraAxe that's really designed for high-speed event-triggered photography, but it's got an intervolometer function where you can specify a number of HDR stops, such that you'll wind up with a bracket of photos (up to 10), though you're limited in how the shots are exposed (the bracket is computed by doubling the Tv for each shot in the bracket).