The general impression I got from the discussions at "that course" was that you must leave a few inches at the top of the under-seat stowage to allow the crumple zone to collapse.
So putting the DG unit, Newscopter electronics or baggage there is OK but don't fill the space up to the top. If you look closely at the seat unit you can work out how it is supposed to deform to absorb energy. Myself I think about 5-6 inches space would work for me.
The Clipper 44 has a 5000 psi helium bottle under the pax seat. I usually wait until after the flight to inform the occupant.
