Let's see how fast Amazon gets me the Hollow Earth Expedition Rulebook. In the meantime, I've looked at the 2008 and 2009 Free RPG Day Quickstarts for HEX, and I must say I'm really impressed. It looks to be a truly quick and easy system to run.
I'm hoping when I have the rules in hand I'll have a better idea if this can be hacked (I know there is a 3 Musketeers version out there) into sci-fi space opera or even high fantasy. I need to look closer.
Still, I like what I see thus far.