Rick,
IF this is an HL/7 (or other structured message) and IF you are placing your custom message type in the appropriate location in the message, you can simply do a TrxId for HL/7 (for example) and add your custom message type to the routing list.
No Tcl needed.
Jim Kosloskey
email: jim.kosloskey@jim-kosloskey.com 30+ years Cloverleaf, 61 years IT – old fart.