I love the TC80N3 and use it all the time but as you say, it doesn't allow for mirror l/up.
A little trick I use is to set the 20Da's self timer on (2 sec delay).
Now set the sequence of exposures on the TC80N3 you want to take but take into account the 2 sec delay and add it to the length of each exposure.
You will hear the mirror lock up, the little white light will come on for 2 sec and then the exposure will commence.
Just allow sufficient time between shots for the camera to settle and you're away.
