Illusion has mentioned the phase change issue which is the main problem of filling a mag with HPa - lack of shot capacity. The 12g CO2 cartridges used with CO2 GBBR mags do hold liquid CO2. Liquid CO2 has a density of 0.77g/mL at a pressure of about 800psi (~56atm). At this density, 12g relates to about 15.6mL of liquid phase CO2 which I find consistent with the small size of 12g cartridges which have an external volume somewhere around 20mL (cross section area x length). If your curious:
http://en.wikipedia.org/wiki/Carbon_dioxide
The other issue is being able to swap mags. GBBRs typically get their gas from their removable mags so you find yourself either modding mags with hicap mechanisms and running gas into the mag body a la
http://www.e-rivera.com/hpa_project/ and having a rifle which is pretty much married to it's single mag. Alternatively you could swap mags and deal with having to moving around a QD fitting with each mag change to conduct gas through each mag which also is rather stinky.
Perhaps a neater solution would be to modify a mag by cutting off some of the top portion, including the fire valve and upper features which relate to mating the top of the mag with the nozzle pneumatics. Permanently mount that stub mag into the receiver and modify a pile of mags to be received which do not have the pneumatics stub that you permanently mounted in the rec'r. It'd take a pile of work, but you'd have a rifle that you could swap mags and run HPa without moving connectors around.