Another thing
"I don't think construction of such a system is going to be that much easier than simply building a first stage, or air-launching"
I like air launch concept in generally, but air launch is limited by size of mothership. I doubt it is very realistic to launch a rocket say +300 tons from any kind of mothership [which would cost say in the range of tens of billion or less to build]
Whereas there is no limit to size of rocket you could launch from a pipelauncher [ you *could* launch the nuclear Orion spaceship if you wanted to- though maybe a pipelauncher that big would cost 1/2 billion or so- but anyways, a Saturn V is quite possible]