OK. I have investigated further and have hit a brick wall. The problem is not the exception being thrown by getFullPath. Both the Linux and HP-UX versions should throw the same exception under the same conditions. The difference is that for some reason, on the Linux system the "base pathname" exception is caught and translated into a "could not open" exception.
I have not been able to find where this translation takes place. I know it has to be in there somewhere, but where? Any pointers? Dudley Irish