I want to turn some CC rugs fire-proof (specifically road overlays). I thought I could easily do that by editing OBJD like in TS3 but that doesn't seem to be the case here.
So how can I do that?

The objd pretty much only controls placement/cost in TS2. There is a bhav for flammability, goes from none to high. Can't say 100% if this is the one thing that works but you could try.

@HugeLunatic
I need to insert it in the init bhav right?

Yep, that's where I've seen in.

Thanks lunie! I've managed to test it finally.
Unfortunately just changing the Flammability didn't work. Then I realized that the previous line is Set Flag "Burns". I changed that as well and it still burned.
So finally I imported the only other BHAV in Rug Globals other than init and main: "CT - Fire - Start". Made it so that it always returns True. It worked!

Beautiful.

Extra notes: I also realized that fire can spread on empty tiles and still leaves ashes behind, whether it starts on the road or just any other empty area.