Success!
Strangely, after playing around with MIIS and AD a bit more, I was able to get changed attribute sources to alter existing users. Here's what I did:
- Using ADMODIFY utility, set a value, such as the word "private" in an unused attribute (extensionAttribute15) for all AD users.
- Using MIIS, modify the source of the homePhone attribute in the SourceAD management agent, so that extensionAttribute15 is the source.
- Run start-onlinecoexistencesync from the DIRSYNC server Powershell.
- On a new browser session, check the user on O365, both in the userlist and Exchange Online address book. The changes should have been made
- Using ADMODIFY utility, remove the value (set to NULL) for extensionAttribute15 for all AD users.
- Run start-onlinecoexistencesync from the DIRSYNC server Powershell.
- On a new browser session, check the user on O365, both in the userlist and Exchange Online address book. The private homePhone info should be gone.
The key, at least for me, is that I was previously trying to use MIIS to point to a null value (extensionAttribute12), which apparently didn't trigger any update to the Metaverse. By pointing to a filled value (extensionAttribute15), syncing, and then clearing the value, the changes were made to all existing O365 users.
By the way... along the way, I found an answer to another nagging question: Why am I sometimes seeing, in the Exchange Online address book, under Availability, "An unexpected error occurred and your request couldn't be handled." It turns out that you will always see this if you are using your tenant administrator account to look at the address book, because when you're signed-in to that account, you're not part of the domain of the user -- so, no free/busy information. Logging-in as a domain user, you should be able to see the availability if everything else is set up correctly.