Thanks for the \out explanation. Seems to be many different ways to do things in sclang.
The control bus is coming from the synthdef defined in this post: Using a boolean value to determine if a ctlbus value is set - #25 by paulelong
I got this working using SendReply, and it works, but maybe there are some intermittent delays. Not sure it’s due to the back-and-forth, but it is seems pretty responsive. I need to do more testing, but if there’s a better way I’d like to know.