Hi Ya
What we used to do as matter of course after every clearance was to spray a water/pva mixture with a garden sprayer to encapsulate any loose fibres that might have occured, and to dampen down the atmosphere, as asbestos is ok when wet. I also had the thought the other day, why noy encapsulate the offending area in Expanding foam. nothing can get out of that. I live in spain, we have a plaster here called YESO, it would be ideal for your problem, it sets rock hard in seconds and is as cheap as chips.

Hope it helps