You're right. I should have tried that. I also find that this information is in the Plug-in developer guide - but still not in the header files, which is a pity, as I expect them to be complete.
There is a curious problem. This works only with the "Unicode file system" according to the documentation, which might not be in effect. Fine if I'm opening the file, less fine if I am working with a file opened by Acrobat.
Yet... I have always passed NULL as ASFileSys and it has always worked. I wonder if the documentation is wrong in asserting that you must use the Unicode file system. Unless, the Unicode file system is absolutely comprehensively compatible with the default, but has extra features, nevertheless allowing it to be used with files opened with the default file system.