This is an old thread from back in November, but I just wanted to let everyone know about my success with this project.
I ended up communicating with Hap Griffen at http://www.hapg.org/astrocables.htm after much searching which took me to a number of astrophotography sites and newsgroups.
For the sum of $25 US, Hap cut and grafted a switchable end on to my TC-80 extension (can also do this directly on remote cord). Very high grade connectors are used,
Great work, works like a champ. I can now use my interval timer functions for the 20D and 300D with way more functionality than just the standard DReb remote control.
If you have a Digital Rebel and want to use a TC-80N3 with it (or have one timer control interchangeable in my case), this is the way to go. Looks factory, and is only about $10 more than buying the parts yourself, which in my case, is worth more than the frustration of trying to get good solder joints and a nice finished cable at home.
Some neat cables and software solutions on the site if you are leaning towards astrophotography.