Hi, that's a good question. If your concern is that you are getting red underline because the syntax is not valid Django, you can avoid that problem by deleting manage.py, which PyCharm uses to assume that it's a Django project. (Actually you would also need to open your project in your file explorer delete the .idea folder to fully erase the existing association.)
As for syntax highlighting and autocompletion, I haven't found a way to do that although personally it doesn't bother me because (1) typing "{{" autocompletes with the closing "}}" and (2) even without the syntax highlighting, it's pretty easy for me to see where my variables are because the "{{" stands out.
PyCharm's django highlighting is convenient but we can't rely on it anymore because the new template language is not fully compatible with Django, so could lead to confusing results, for example many Django template tags and filters don't work, and there are some differences in the syntax.
In oTree Studio, the new syntax is an improvement. It was really quite a pain to type out the {% %} ... {% %} and there was no smart autocomplete for django syntax.
I also thought of some PyCharm extension...that would be nice...i guess it would not be too hard. Maybe an interesting side project for someone :)