adding something to it that wouldn't change how it funtioned
But....! Changing how it functioned is exactly what you are
attempting to do! Is it not?
An ASL should work. If the information is going to be used
immediately, it should work fine. Modifications to an ASL take
place immediately and can be accessed immediately. For that
matter, you could put your information in a quest stage.
Why not put the information in an SA attached to the caster?
This might be better.
What I worry about is that:
1) Cast the spell and select a target
2) Put the information in an ASL
3) Another character casts the spell and selects a target
4) Put new information in the ASL
5) Use the information in the ASL Oooops....wrong information!
6) Use the information in the ASL
Perhaps this is impossible in your case. If it were possible
in ANY case, it would be bad because other folks might copy
your example in cases where it would fail.