A charcoal fired forge is highly feasible.
To concentrate the heat you want you must reduce the sources of heat loss.
Heat is lost radiantly. The forge must allow access for you part and yet minimize heat loss. I believe traditional forges put new fuel on top to insulate and the air is pumped in from the bottom. You could add a lid that not only closes while you are working the piece but also will close on the piece when it is in the forge.
I believe they conserved fuel by letting the coals naturally aspire until they place their piece in them at which time they would pump a bellows with their free hand. I you use a blower you may want a shutoff valve to facilitate this same kind of fuel conservation.
If you should have trouble getting your forge hot enough you could preheat you incoming air. This is the real beauty of solid fuel. You can heat the incoming air as hot as you like if you try that with a gas or liquid fed fire you will get a series of inefficient explosions rather than a continuous burn. If you make a chimney for the forge you could use the heat of you exhaust gasses to heat you incoming air. This is known as recuperation.
Here is an interesting link regarding charcoal fired forges
http://www.moosecreekforge.com/hints.html
Building a forge from scrap
http://www.survival.com/forge.htm
Some information on making your own refractory materials from more common components
http://www.backyardmetalcasting.com/refractories.html
