Redwall Wiki editor and member of the online club Dibbuns Against Bedtime Dorothea Duckfontein Dillworthy (also known as Dotti) recently wrote a letter to Brian Jacques. Mr. Jacques apparently responded rather quickly, and the following tidbit of information was revealed:
"[The Dibbuns Against Bedtime] will be appearing in my next book to amuse and confound all the abbey creatures again."
As Brian also stated he was about halfway through writing this book, it's very hard to ascertain if he was in fact talking about The Sable Quean, considering this novel must be in the hands of Penguin right about now. He may very well be referring to a future Redwall 22. Either way, this is exciting news.
Thanks for the tip Dotti!