Well. I started something like your Mod a loooong time ago, It was called Invasion 106, now it is a dead project, I didn't stoped because of the limited old Engine, Really, I made some HL2-Esque maps with the max of 1000 R_Speeds, I stoped because I just don't wanted to get in trouble with legal issues. For who those say HL1 engine doesn't support HL2 complex maps, here's some proof that it can handle HL2, just better than HL2, those maps have max of 800 R_Speeds and a P3 500 mhz with some 64 mb video card would get 30 Fps or more.
Lol yes, its Goldsource.
Those screenshots are from a mod I'm working as a mapper, it still in active development, we just got 2 new coders showing screenshots like that, and now 3 mappers came on the team too.
The mod's name and development place I'll not reveal now. For people who will say that its illegal, Its not, we got a free way to get our content, it look like your HL2, but its not. You'll understand some day.
Cheers.