It would be most helpful to be able to use the Constant part in fetch alias definition for attribute_filetr.
Unfortunately it does currently not work, as recursive arrays are not supported for values assigned to Constant params.

Suggestion: instead of using a simple split(';',$val), we could use a json_decode($val) instead, so that it would work with arbitrary nested values

[obj_by_attr_val]
Module=content
FunctionName=list
Constant[parent_node_id]=2
Constant[attribute_filter]=[["classname/attrname","=","value"]]

Even better: allow the parameter fed by user to be set inside an array

Parameter[attribute_filter]=[["classname/attrname","=","$
userparamname"]]