Well the BB device in question has been working for the past day now
Here are the details of what I did:
1. Used a spare BB device that was assigned to another user (BB 8707, O/S version 188.8.131.520 (English-language)).
2. Run the Erase Data and Disable Handheld from BES Manager (the device had been assigned to another user).
3. Delete and recreate the user.
4. Applied a policy to the BB device with the setting "Disable Wireless Bulk Loads" set as "False" (previously it was not set to anything).
5. Initiated wireless activation.
This time the activation completed in about four hours. Actually, all items completed synchronization except for contacts (believe that's because the user has photos attached to some of his contacts), so no "sync complete" appeared on the BB device itself, but I did see the *** SLOWSYNC COMPLETE *** log entry in the BES server's SYNC log.
I am still not 100% sure what made the sync work this time around. The only things that were different were the O/S version (but activation with the same O/S had failed before) and the wireless bulk policy setting. The article that freakinvibe provided referred to the wireless bulk policy setting, but I thought that the default setting (that is, no explicit setting=enable bulk sync) was sufficient. It would be nice to have something like a "gpresult" tool for BB devices...
In any case, docomo BB support said that they would escalate this case to RIM support. I suspect that we will be advised to improve connectivity between our BES server and our Exchange servers (as the ping times between the two are in the 70-80ms range). Something else that we'll look into is upgrading the BES Manager (currently at v184.108.40.206).
I'll update this post again as we get more info from BB support.