Named Parameter Markers in DB2 for z/OS
First of all, the syntax for a named parameter marker is CONCAT(:queryDate, '-12.00.00.000000'), that is, it begins with a colon, not a question mark.

Processing of parameter markers is performed on the client side, not on the server, and it's irrelevant if your server happens to be DB2 for z/OS or not. The IBM Data Server Client (the common driver for IBM database servers) supports named parameter markers. 

Whether Data Studio SQL editor can properly parse named parameter markers and pass them to the driver is a different question, to which I'm afraid I don't know the answer.