​Here is a patch that attempts to implement Juri's approach.​

​However, at the moment it doesn't work. I think the problem is to do with combining process filters. As suggested in the Emacs manual, I use add-function to combine the new process filter with the default comint-output-filter. But no matter what I try, the original filter does not seem to be run any more, even if I pass a trivial function to add-function. I've studied the documentation, and I can't see what I'm doing wrong.