It does look like CHGMEDBRM VOL(MYVOL) EXPDATE(*NONE) will expire the
volume, at least according to the help. And the messages you can monitor
for on that command do not include the usual messages like
BRM0010 40 Volume &1 cannot be expired.
If that command is used by option 7=Expire of WRKMEDBRM then it would be
quite easy to write a command exit point program to lock it down to