The answer is simple:
The "#driver.mbx/" prefix is a recognized only in the CREATE and (in the case
of zero-length files only) APPEND commands.
It is not part of the mailbox name. Nor is it (contrary to what you assert in
your message) part of the UW specification of a mailbox name (which is in
naming.txt).
The "#driver.mbx/" prefix is only mentioned in the FAQ:
(1) The simplest way to create a mbx-format mailbox is to prefer the name
with "#driver.mbx/" when creating a mailbox through c-client. For
example, if you create "#driver.mbx/foo", the mailbox "foo" will be
created in mbx format. Only use "#driver.mbx/" when creating the
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
mailbox. At all other times, just use the name ("foo" in this
^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
example); the software will automatically select the driver for mbx
whenever that mailbox is accessed without you doing anything else.
Finally, the "#driver.mbx/" is not part of the IMAP protocol. IMAP
specifically shields the user from details of what format a mail store may
happen. The "#driver.mbx/" prefix is specifically a power tool for people who
know what they are doing.
