I agree that the official function plugin should keep using "number" index, since it makes the interface simple. "expression:Call" could not use "string" parameter as well.
To support "string" index, a solution is using my hash table plugin or official dictionary to transfer parameters into JSON string and set this string in parameter of official function. In "On function", unpack the JSON string by hash table plugin or official dictionary. to get parameter by string key.
(string key, value) -> hash table -> JSON string -> function parameter
-> "call function -> "on function"
-> function parameter (JSON string) -> hash table -> (string key, value)
<img src="http://i1081.photobucket.com/albums/j352/rexrainbow1/FnHashtable.png" border="0">
Edit:
I update my hash table plugin to support create JSON string in an single expression.
<img src="http://i1081.photobucket.com/albums/j352/rexrainbow1/FnHash.png" border="0">