Sounds reasonable to me, can you submit a patch (I think for editor.h and highlighting.c but do check if anything else is needed).
I have attached some patch, I hope it is enough.
PS and if you want to add the ability for the standard system to have another regex for differentiating warnings from errors that would be even cooler, but a lot more work.
Do you have in mind building? From what I saw in code (mind I don't build anything usually, I am mostly Python programmer and if I build I just run make from the terminal), the red indicator is used to mark some errors in code. I understand that Geany parses output of the compiler and sets indicators in proper places? So I could also try to parse warnings in the compiler output and set those indicators there?