Although we are used to using {}'s with arrays, I don't think that's a good idea in mIRC because {}'s have their own meaning and I think it could become a problem if we use them for arrays. I'd stick with just ()'s like any other command. It would be fairly easy to parse the following without needing to use {}'s.
/aset <aname> ((value,value,value),(value,value,value),....)
Or ;'s can be used between them (with or without the ()'s as shown above)...
/aset <aname> (value,value,value;value,value,value;....)
I just think it will be better overall to avoid trying to use {}'s for this when it will require a lot of changes (I believe) to the parser to know how to properly treat them and when to change indenting.