I do a good bit of this with a variety of both fixed lens digicams and dSLR's. I really don't think you will be satisfied with any afocal solution (shooting through an eyepiece) with your dSLR.
Typically to shoot through an eyepiece you need a relatively tiny lens. The very best digiscoping candidates have traditionally been the little CP series Nikons. Personally I use the CP950, CP990 and CP4500. With the 4500 it's possible to get decent shots at up to 6000mm with good light. But to use a scope for your dSLR you are "MUCH" better off either using an inexpensive astro type (I recommend the Meade ETX-90, etc.) or using a specialized and dedicated front end (lens) to connect to your camera with a spotting scope.
Personally I use two different arrangements for this. With my dSLR's I use either the Meade ETX-90 which has a camera port, or the Swarovski ST-80 HD with Swarovski's own proprietary front end.
For the Meade you simply purchase the Meade "T" adapter and use a standard Canon "T" connector. You mount the Canon "T" connector to the camera, mount the Meade "T" adapter to the camera port on the Meade and this gives you a fixed focal length ot 1250mm at F 13.8. With the 1.6x reduced FOV on your 20D the "effective" focal length will be 2000mm. You will loose autofocus and you use the scope's focus adjustment to do all focusing. With good light and a sturdy tripod you can get quite nice frames but I also suggest using mirror lockup (the 20D has this) and a Canon remote release. First press locks up the mirror, second press trips the shutter. You need to experiment a bit to get optimal results, but the mirror removes the possibility of chromatic aberration and you can actually achieve excellent results with practice. A side benefit is that the Meade will focus as close as 8 feet so you can actually shoot the equivalence of macro's this way. I have a few samples if you're interested in seeing them.
The second method is to use the Swarovski's dedicated front end which is actually a lens which also mounts to your camera via the Canon "T" connector and replaces the eyepiece in the Swarovski. This comes in two "flavors" - 800mm (plus your 1.6x FOV crop) and 1100 mm. Of course you loose atofocus and again must use the scope to do your focusing. The 800mm version gives you F10 and the 1100mm gives you about F14.http://www.swoptics.co.uk/view.asp?KEY=1441http://www.buytelescopes.com/product.asp?t=103&pid=1858&m=
There is, of course a large difference in the costs of doing it these two different ways. My Swarovski cost me about $1600 with a 20-60 eyepiece and my 800 mm adapter was almost $400. The adapters I linked to above are less expensive (they have come down with increasing demand).
You can sometimes find a great older model Meade ETX-90 for around $200 and the Meade adapter will set you back about $50 and an erecting prisim another $40 or so. So you can be digiscoping with the Meade for a total investment of around $300-$400 and for the Swarovski for probably $1800 or so.
The major difference is the ruggedness of the Swarovski. It's waterproof to something like nine feet, has nitrogen filled lens barrel so no fogging up in inclement weather or temperature changes and can take a lick. The Meade, like any mirror type scope is much more fragile. From an optics perspective it's really a wash in my experience. They each have excellent optics.
Let me know if you want links to sample shots with each and I'll dig them up.