Some more experimenting has convinced me that the file lock is released by the termination of the *LAST* program to access it.

This means that you can release any lock by opening it with something you can then close--this will free up the file even if the original lock was due to Windows.  Note that what you open it with need not be able to understand it--I used the file viewer in Total Commander as it can open a large file without reading it all into memory and won't puke on binary data.