Moving the mailbox to another message store seems to have slowed down the problem.
However, the move did not work the first (or second) time, and failed with the following error:
<summary isWarning="false" errorCode="0xc1054005">
The MAPI call failed. MAPI or an unspecified service provider. ID no: 80004005-0000-00000000
I went looking in Microsoft Technet and found the following article which appeared relevant: XADM: You Cannot Move a Mailbox, and Error Messages That Contain Error Code 0xfffff9bf Are Logged in the Application Event Log
However as our Exchange Server logging was set to "none" for all diagnostics, the event log messages indicating which folder was causing the error in moving the mailbox was not there. So I set Mailbox Move Diagnostics to "minimum" and tried moving the mailbox again. And lo and behold, this time the mailbox moved without any errors, and once the BES had reconnected to the moved mailbox, normal Blackberry/BES service resumed, and the 20452 errors stopped - for about an hour or so.....
I cannot explain why the mailbox would not move with logging set to "none", or why the mailbox moved and cleared up the potentially corrupted hidden folder when logging was set to "minimum".
The problem is still there, however the rate has dropped dramatically. Now we are getting the 20452 errors at around 1 every minute or so (before they were coming in every 2-3 seconds)
I do wish the original event was a bit more helpful in indicating what folder it is having trouble with, instead of just giving a meaningless 6 digit number.