I read your earthbagbuilding website and it was very interesting. It made me think of something that my grandparents did back in the 1970's in a completely different application.
Back in the 1970's, my grandparents had a very modest lake cabin (a shack almost) on a fairly decent sized lake. They had problems with shore erosion and they decided to build a "seawall". They looked at several different construction techniques, mainly things based on cinder block construction (we had lots of family members who were concrete guys and masons). The problem with a cinder block sea wall is you need to put in a pretty deep foundation otherwise the water will eventually undermine it and the wall will collapse into the water.
About that time, they noticed a sea wall down the lake that seemed to be built out of exceeding regular sized rock. On closer inspection we saw that the rock was concrete in these fairly uniform sized pillow shapes.
We talked to the property owner and he revealed what he had done. When the lake was let down annually he had simply stacked 60lb bags of quikrete into a wall. When the water level was allowed to rise, the water seeped into the bags, and hardened the concrete into interlocking concrete pillows. After awhile the paper bags deteriorated and were washed away. It left a very heavy, very solid, and good looking "rock" wall when finished. Apparently back in the 60's and 70's Quikrete actually had a little "how-to" on how to build a wall in this fashion. The interesting thing was that allowing the concrete to set like this (i.e. concrete powder in a bag with water soaking through) actually produced a much stronger concrete than what you get with traditional methods. Also, according to my memory of the quickrete how-to, this method was not limited to sea walls. You could use it anywhere and simply hose down the stacked bags every day for some number of weeks and get the same results (and in fact, I've seen unopened bags of quickrete left for long periods in damp environments like under a crawl space and indeed it does harden eventually into a very hard bag shaped lump of concrete).
It made me wonder if this technique could be combined with your earthbagging technique. It might be especially useful in areas where good bag fill material is hard to come by (where I live in the southeast most of our soil is almost 100% red clay which is a terrible fill material for earthbagging). Also, one would presume that the hardened concrete bags would be both denser and stronger than an earth filled bag, which would be useful in a fallout shelter situation. I've started to look around the Quickrete website to see if they still have a how-to or whitepaper on this technique, but so far I haven't turned up anything.
I haven't run up any figures on the cost, so that may or may not be an issue.