Pause render updates (preview Lock button)

[Kamen Lilov]

Hello,

I'm looking for an undocumented feature :) - in the SI preview window, 
there is a Lock button that will cause SI to stop sending any further 
Render requests (for that preview) until the button is toggled again.

Is there a way to programatically access, and change, the value of this 
toggle? Or at least to simulate a user clicking that button? I suspect 
that it all comes down to a flag private to the preview window 
implementation (it's not even a scene or pass property) but I need to 
confirm this.

K.