And like all very good questions, it's difficult to answer! In short, I don't have a definite answer on this one.

However:

There were plenty of "conduct books" around that laid out many rules and regualtions about what one should, or should not do. However, my very limited knowledge of these books doesn't include much about how to eat. Neither do I recall anything written in a novel about eating with one's mouth full from which one could extrapolate a rule about whether it was , or was not acceptable. The only thing I can recall off hand is that ladies were supposed to pick at their food, and not eat much at all.

The total absence of any do's or don'ts in my mind suggests to me that people didn't worry too much about whether one did, or did not, talk with one's mouth full. But I could be wrong on this.