I've got some other things brewing (as you may have guessed) and will fill you in as they take shape.
Doesn't seem very hush-hush, just that there's nothing new to report.
Considering that was posted on the Sam & Max blog and not Spudvision (which hasn't been updated since a month before Sam and Max's last post), I can only guess he means "S&M things" instead of "stuff that's preoccupying my time", but even so, Purcell's a busy dude.
I really, very much doubt we've seen the last of those two in game form, but I also think now's probably the time for patience. Between Fables, King's Quest, and Walking Dead Season 2, there probably won't be any Telltale Sam and Max stuff for a while, and the LucasArts purchase hasn't changed anything, as far as Disney goes.
(besides, when you think about it, the devil's playhouse's ending presents a problem. It'd be weird if Season 4 has Max's powers, since we already had the whole last season's plot revolve around them, and destroyed them. But taking them away would be just as weird, gameplay-wise. So it probably wouldn't be the casual graduation from season-to-season like we're used to.)