Hi Malc,
I have not found a way to do this. As far as I know, you can either use a path relative to the project directory, or a full path.
If I understand you correctly, you wish to have an include directory common to more than one project, outside the directory structure of any particular project. I'm not sure if this is possible without dictating some sort of directory structure to the developers.
One thing you could consider:
EXTERN "..\External\myInclude"
This way you would only have to enforce the use of a single directory which contains both the projects and the External folder. This would work for:
C:\MyProjects\Project1
C:\MyProjects\Project2
C:\MyProjects\External
but also for:
D:\Develop\WDProj\Project1
D:\Develop\WDProj\Project2
D:\Develop\WDProj\External
Cheers,
/sohan