Oh, I see, I thought you were talking about wrapping a filename using JUCE_LIVE_CONSTANT.
Well, it's true I never anticipated that __FILE__ would be anything other than an absolute path, because IMHO it's completely nuts for a compiler to produce a partial path.. Within a piece of compiled code, there's no way to possibly know what that path should be relative to! All other compilers I've seen use absolute paths FWIW.
But this actually proves my point.. There isn't a correct "fix" for this case, because the running code can't possibly know where the original source code was compiled from, so it's impossible to supply a correct base path from which this path could be correctly determined. So the correct thing for the program to do is exactly what you're seeing: i.e. a run-time assertion warning about undefined behaviour, rather than having some kind of hard-coded fallback path in the code which may or may not end up resolving to the right file.