[racket] Test for async-channel being full

I'm trying to make a custom output port that is backed by a
async-channel. One of the operations that ports need to provide is an
event that is ready when progress on the event can be made. But since
I cannot tell if the async-channel is full and put would not block
without actually sending a value, I cannot provide such an event. Is
it possible to provide such an API or does the implementation of
async-channels prevent this?