Edit the routine of an unused event command (or one of the variants that you never will use). Though TF won't know what your routine does, it will still insert it into the event as the old command.
Use the Event Commands document in Geiger's offsets archive and find an unused command, and then find it's location in the NA Offsets document to edit it. Then use that command in TF to trigger your code.
Example: Command 7A is unused. You find Event Command 7A in the NA Offsets document, insert your code over the unused 7A's routines. In TF, use Command 7A to trigger your code.
7A probably isn't unused, but this method would be my best guess on how to go about it. The Overworld Events has a command that jumps to custom routines, if we could hook into that somehow, that would be neat.