Great idea for a Theme Component!
I’ve gone ahead and submitted a PR to do the following:
- removes need for jQuery
- removes need to worry about timing (use Ember framework & Discourse core, don’t fight them)
- modifies the Composer Model to help achieve the result (as you rightly suggested)
I won’t be upset if you don’t merge it, but hope it helps to see another way of doing things 
https://github.com/naidihr/discourse-topic-template-placeholders/pull/1
(I can’t promise there are no bugs, but happy to follow up on any ! 
)