What you have won't work because it is what doesn't work and is what this thread tries to resolve :)

A bang should take a fixed number of arguments if you're using \1, \2, etc, right? Something like this should work:

!varset a1 "%{\1}"
!varset a2 "%{\2}"
...

What you have is too complicated, because there is no variable that stores the number of arguments passed into a bang.