I'm not an expert in these matters, but would I be right in thinking that if HL requires a P166 and 64MB, it would be possible to run it on a G3/G4 under emulation, if such an emulation solution exists of course...
I suppose it might work in software rendering under Virtual PC if you run Mac OS X on top of Mac-On-Linux under Debian on your A1 / Peg.
Frame rates would not be anything to speak of though...