Hey!
So, in the example folders there's a script called "STMPreparse3.cs" which I think does what you're after. (The preparsing scripts' Parse method should be invoked with STM's OnPreparseEvent in the inspector.) This one example script demonstrates a <transcribe> tag that changes all instances of "A" inside it to "aaa" and all instances of "B" inside it to "bbb" until it is cancelled with </transcribe>, as an example. In reality, this type of thing is really useful if you have something like... a custom event tag you'll be typing a LOT. You can have "<g>" get replaced with "<e=myReallyReallyLongCustomEvent>" and STM should know what to do with it.
I hope this works for you! What did you have in mind for your project in terms of custom tags? I can help write up a custom preparsing script if the example doesn't work for your situation.
There's a lot of functions like this that STM has that are hidden away in additional components... (ruby text, alternate outline generator, pagination) I'm thinking of making these system either easier to use or entirely built-in to the STM components for a future major update, but that'll probably be STM 2.0! Someday...