Using VBoxAddtions-os2-x86-5.0.51.r217, I found the following
I can drag and drop files. If I do, it moves rather than copies files. If a file conflict, it behaves correctly.
Resulting file dates are garbage. Copying a file results in the file in the shared directory with a file date in 2099.
Creating a folder on a shared drive results in a Folder with unusable permissions. You must return to Linux and set the group permissions to read+write to use the folder from OS/2. Dates are screwed up as listed above for files.
Deleting files and folders works.
Lots of warning that extended attributes are not supported.
copying from command line works, even if overwriting, but file dates are still wrong. Both directions seem to work.