Thanks to the two responders for their thoughts.
I've done some experimentation on this and the results are that you can use the "note off" velocity map to interesting effect even without a keyboard that sends note-off signals. This also suggests that note-off velocity has a definite impact on the piano models ( and hence the sound you make ).
It turns out that ( for keyboards that don't send note-off ) Pianoteq seems to always generate a "fake" input note-off velocity of 63 or 64 ( my visual estimate on the note-off map display ).
But you can alter this simply by changing the note-off map curve. A value of zero leaves you with a sustained note and a higher value means a faster dying of the sound. Basically you're going to be mapping the constant input value that Pianoteq generates ( that 63 ) using the map.
So rather useful to know if your keyboard does not support note-off, as the map does affect the rate the note dies at if you know you can make use of the thing, perhaps using the map to affect the way different pieces you play sound.
This also means that the models must actively use note-off velocities to control the rate the note dies at when keyboards support note-off. This does seem rather important for more serious players. I've no idea how closely this reflects how pianos work in practice, but I'd assume Pianoteq didn't do this by accident.
The note-off velocity also seems related to the generation of a key release noise ( which is documented in the manual ). I often mute or reduce this anyway ( not being from an acoustic piano background myself it's just distracting to me - I'm sure some people will be horrified by this, not least the people who implemented the code :-) ).
As I had (stupidly) ignored the note-off velocity map when I found my keyboard did not send note off signals I apologise if I wasted anyone's time with this and hope this information will be useful to someone. Also note I'm using STAGE 5 so I can't be sure this is valid for any other version.
StephenG