Interesting Richard, the SpeedyMelt furnace at the college was about the size of yours if not a tad bit larger, it was a natural gas furnace.
I don't see your bronze as having gas holes/defects, I don't think there's a cast made that comes out perfect, they all need some fixing and the like.
The instructor at the college was Ted Isto, I see he is still around with the arts group. We looked at a new thing back then, a ceramic shell system by Shellspen, it looked great but then we read that you had to keep the slurry in a constate agitation or it settled out and solidified, fine for a foundry using it all day but not for what we or I myself would be doing.
I discovered they are still in business, and now have a kit which seems like it would be ideal for casting a few small pieces at home. They apparently solved the agitation problem with a formula that doesn't have to have that done.
Check this out;http://www.shellspen.com/metal-casting-kits.htm
The $149 kit has enough to cast 11 pounds of bronze, but that may be optimistic, so it might realistically be 7-8 pounds. That would be enough for one of my dog casts, and with the weight of a cruicible, tongs, and the total metal, that's probably all one man could manage alone without a tilting furnace, or overhead trolly system.
Their $229 kit is probably more realistic, and include enough "extra" materials to allow for a wax that comes a little too close to, or slightly over the size limit the smaller kit would properly cover, and allow for some waste/loss/spillage.
So with the ceramic shell part apparently solved with one of those kits, the other issue is the furnace and a burnout furnace.
With the brittania metal dogs I made, there was no wax to burn out since the mold was a plaster-sand piece mold, so only the water had to be removed.
Propane would be the viable thing here.
I know as you wrote- the usual method is an oil clay model, rubber mold/plaster shell to cast the waxes in, but at the time I wasn't confident with the mold rubbers I was using to do the mold of a standing four leg figure and deal with all the shims, not tearing the rubber etc.
In my case, making a plaster piece mold was quite easy, blocking off one section at a time with water clay, and filling it with hydrocal, no shims needed.
Most of them wound up being about six pieces- four smaller pieces and two halves.
I used Polytek urethane for the rubber positive block molds, I have them and they are still in excellent condition and can be used to cast some sand/plaster piece molds from.
One of these days I want to take some waxes out of them.
These days I use Smooth-on Rebound 25 platinum silicone rubber, the stuff is great, very soft with shore 25, and very, very strong- unlike almost every rubber I've used (except latex) but it's $200 for the two gallon kit, and for most of my models it takes the whole kit.
I have a kit on hand now and need another one, as I will be making two molds this week and when I order another kit.