Heh. If it helps, I managed to take garrison archers with me once.
They messed up my units list and counted like killed solder for "give art" action.
One was killed and another left an army in a couple of turns later.
Even I managed to reproduce it, but I am not sure:
1. have creature at garnison.
2. survive the siege.
3. make clicks on empty portraits using shift
4. get free archers.