Method to block Jupyter continuing to next cell?

I'm trying to do some trickery using Comm objects: a cell in a notebook sends a message to the frontend, which sends back a reply. I've figured out how to asynchronously handle the reply (so that it doesn't block the event loop from receiving the reply), but I'm having the problem that when telling Jupyter to run all, it starts executing the next cell before the callback has had a chance to run.

Is there somewhere I can modify the busy/idle handling so that the kernel doesn't indicate that it's ready to run the next cell until I manually indicate idleness in the callback? That is, assuming it is the idle notification that triggers the start of the next cell, rather than something else,