Thanks for all the info on regulators, etc.

Now, I'm looking for burner designs.  Something that would work with the small propane/butane canisters used for backpacking stoves seems like the way to go, just based on convenience.  

The boiler I'm considering is a horizontal style having a cylindrical firebox penetrated by water tubes.  The burner drawn on the plans looks rather like a small propane blowtorch burner, but doesn't have any dimensions.

The commercial burners I've seen seem straightforward enough, but I understand that choosing orifice sizes, etc, involves a bit of black magic.   I'd rather benefit from someone else's experience here.

Of course, if you have an argument for (and proven design for) some  alternative fuel like white gas or denatured alcohol, I'd love to hear about that too.

-Matt