Hey!
Check out the "PreParsing" feature of STM. I should have a sample component included called "STMPreParse" that should do exactly what you want. This component will add a specified string of text to any object if its "Parse" method is invoked by STM's PreParse event (It's one of the UnityEvents under the events tab!) Just set the string to <w=custom> and it'll apply that to the mesh when you change its text!
You don't need to use </w> at the end of the string, but "STMPreParse2" can actually do just that, too! Just make the UnityEvent invoke both!