Honestly, I would just use the standard gaps and cracks stuff. It ends up forming a denser foam that seems to adhere a bit better. Sprinkling stuff on the surface right after application would probably not work as it would expand and expose gaps in whatever you sprinkle on. So you will need to coat it in silicone (or dryloc or Titebond 3, etc) anyway and then add to that your "dust". Which could be coco coir, peat, or tree fern fiber among other possibilities.
As far as accessing the pump, I have no experience there. I would probably try to set up some kind of sump and avoid leaving the pump in the tank at all.