OK, perhaps the feature went away with two-way remote (which is funny, since the two-way IR hardware is still there). Darn, I have a 2-way LX6000 I could try, but it is stored in an unaccessible place right now...
Inserting a headphone mutes both internal & external speakers. PL speakers ought to go on playing, since it's just a mechanical switch inline with the passive speaker outputs, the rest of the TV knows nothing about it.