Had a thought a while back & finally tried it out.
Got hold of an old 2.5KG CO2 fire extinguisher for £6. Removed the 'horn' & replaced it with a section of hose from an old track pump - attached to which is a Topeak Super Chuck thingy.
Removed the safety 'plug' from the filling socket, drilled a 6mm hole through & then slid a presta valve from an old tube in. Screwed it back in and bingo, a steel container with the ability to be pressurised with a track pump and a trigger release.
A couple of test runs later & it seems 40psi in the tank will seat a 2.1 tyre & inflate it to about 25psi - 100 psi in the tank gives enough to seat 2 tyres one after the other.
I'm really quite pleased with it.
Once they're seated, simply top up wit a track pump as usual.
I am tempted to fit a pressure gauge and use a much higher psi to get rid of the need to adjust tyre pressures after though....