Bill, I use the fire retardant caulking that building inspectors require for plumbing, heating or electrical penetrations. It is good for 1200 degrees, comes in a caulking tube (about 9 bucks, and two tubes will do every opening you need to repair. )
I've put a torch on it and it won't burn, only give off a nasty smell.....so no fear of burning it out in your application.
Get a pair of light duty throw away gloves and caulk you way around the perimeter, smoothing the bead as you would on a sink. I never hardens completely, is striated with some sort of fiber that makes it super strong and flexible. You can easily do around openings, removable side openings, etc and when you need to remove it, sharp putty knife. I've picked up a base and boiler caulked together and they haven't separated yet. I use it on the AA's and AHS's too, simply because it stays flexible, is inspector and fire dept approved and workable when it is applied.
If you have a few tubes left over, we'll send them to Richard for the rebuild. I know he will use them for any mechanical openings he needs to seal, and everyone will sleep better. I know that old balloon framing will be fire-stopped next time.
Thousands of houses here that are framed the same way, even my own was. I gutted the inside (1890 High Posted Cape) and re-framed and fire-stopped between floors. Everyone who works on old house knows the possibility of cellar to attic passageways in these homes. I also did dense-pack cellulose everywhere, so you may have other purposes for this product also.