Hi there,
I have a newly self-modded 40D and have used the remote shooting. As long as live-view is activated, then you can go with the timer and still have MLU - since the chip is reading light data, then the mirror is already 'locked-up', so no need for MLU. But if live view is off, then the TC80N3 is the only way to do MLU shots.
Regarding ISO:
There is a school of thought that because of the 14-bit light collection (never, never use jpg that only gives 8-bits of data and with artefacts), the Canon reaches 'gain-unity' at ISO 400, so from a astrophotography point of view you never need to go above this. For a 20/30D I think the figures are around ISO 1000 (only possible on the 30D - 800 or 1600 on 20D etc).
For back-of-the-camera colourful shots, however, you need 1600 and 3200 to get the intensity to get good colour - great for star-parties.
Regarding Live View and Noise:
No problem from increased noise with LiveView - since we are always taking sequences anyway, and unless you deliberately keep 30sec between frames to cool the sensor, then don't worry.
If anyone wants their 40D modded, or wants an older 20D already modded, just let me know
Regards
Clinton