Since I am applying for STS but semi-finalists will be anounced Jan. 7th, is it worth it to mention that I am at least applying for STS on my apps?
You should talk about your research in your application, and mention in passing that you will be submitting this to the Regeneron competition.
And you can update the colleges if you get selected as a Scholar, and again as Finalist.
My daughter didn’t mention it (probably thought she’d jinx herself) but it’s reasonable to do so as @hebegebe mentioned. Good luck with it and with your apps!