POM is already possible via autoexec.cfg, afaik. No need to add it anymore.
It has no effect in-game. Try it and see for yourself

Maybe with patch 1.4. But I'm playing SP with the exe from 1.2. Maybe I'm wrong, and it's only a illusion, but I have the feeling, that POM works....and in the console the game accepts "r_UsePOM 1"....
Yes, it accepts the CVAR. Doesn't mean it works. Do a screen-by-screen comparison of some terrain if you don't believe me.