EFS encrypted Office documents lose additional users after saving the file

I create a new Word document in 'Office Open XML' format (docx). I then encrypt it using EFS and add several other users to access the file.

Now I open the document and edit it, after saving it, the file is still encrypted but the additional users are gone! This means I have to remember which users I gave access to originally and add them again every time I edit a document.

If I do the same with a different file type, say *.txt, the additional users are kept after editing.

I assume this happens because the new Office documents are zip files that are unpacked and then re-created rather than just opened and closed. Word is smart enough to apply EFS encryption again, but only for the current user.

Does anybody know a way around this?

We are using Office 2010 on Windows 7 with AD based EFS.