Sorry, but you are wrong. 20Da has a special type of sensor that allows the use of live view. The live view function in 20Da is crumpy. The function found in new cameras (40D, 450D, 1000D, 5D mkII) is way better and more flexible.
Also the IR-filter is not just "added". All digital cameras require a filter in front of the sensor to block infrared light. Unfortunately for astronomers, hydrogen nebulae emit light at a wavelength (color) of really deep red, almost infrared. This wavelength doesn't make it completely through the IR blocking filter in normal cameras, only 15% gets through.
The IR blocking filter on the 20Da is merely different in a way that it let's a little bit more red through, allowing this certain wavelength to get through completely. 20Da is also able to take normal photos.
So you can't make 20D to become a 20Da by any means, firmware or hardware. You can open 20D up and replace the filter with a more-red-permitting one (these are sold by Baader Planetarium for instance), so you'll be able to better shoot nebulae.
You can't get live view with anything less than buying a model that has live view.
