>>the ‘all controllers off’ triggers some old memory that this may not always produce the desired result. Any chance that this could be optional in some fashion?<<
are you sure you’re not thinking of "all controllers zero"? if the function simply stops any active controllers from transmitting & leaves the controlled device parked where it was, then it would be left to the user to restore normality by re-loading his patch.
but if you send an "all controllers.." message with a value, then some devices will do mad things. I have many modules here that react badly to controller values being sent to zero because the affected parameter actually has 63/64 as it’s "zero" & uses 0-127 in a bipolar fashion.
the yamaha qy70 sends a "set to zero" message on cc66 whenever a pattern is started. I have no idea why- maybe it’s a general midi or XG thing, but it means I can’t use the qy70 with my supernova mk1 unless I shut off osc-3 in every patch.
does anyone else think this is a can of worms? 
d.