Did some research most of today to find out that this is also called pass parameter as reference. From what I can tell, this is impossible in Filemaker.
If there is any other than IMPOSSIBLE, let me know. For those of you following along, I've learned that Global Fields are the way to go in Filemaker.
I think you want to pass the Global Variable's name and it's value as parameters:
Make this the expression you pass as a parameter: List ( "$$GlobalVariableName" ; Value )
Your script can work like this:
Set Variable [$Dummy ; value: Evaluate ( "Let (" & GetValue ( Get ( ScriptParameter ) ; 1 ) & " = " & GetValue ( Get ( ScriptParameter ) ; 2 ) & " ; 1 )") ]
$Dummy receives the value 1, everytime, but the indirectly referenced global variable is assigned a value passed as a parameter as a side effect of this expression using Let to assign a value to the global variable named as the first parameter.