Technically you CAN pressurize a liquid (think hydraulics), but the pressure is nearly instantly relieved upon expansion. Liquids don't compress very much. Gases compress very, very well and can be used to store lots of energy. So when a DE bell full of pressurized liquid releases, there is very little stored energy. That's why tanks are hydrostatically (read: with water) tested and never tested with gas. The first few drops of liquid to leak relieve the pressure. That said, I use CO2 to push the last bit of beer through the filter. It's always fairly low pressure as I just keep enough pressure on the filter to keep my beer from foaming. Breweries are not inherently dangerous places and DE filters are less dangerous than automobiles; you just have to keep your wits about you, work thoughtfully and carefully, and use well written SOPs to do your work. Good luck and work safely!
Phillip Kelm--Palau Brewing Company Manager--
Worldwide Brewery Installations