I DID IT!!
Microsoft has just the tool for doing this, and it's free. XML Notepad 2007. You can download it from the official Microsoft website.
Here's what I did: (In all cases, where I'm changing settings, I'm using a browser to the device
192.168.x.xxx, NOT Obi Expert)
1) Made backup copies of my settings. (Don't select "Obi Version," you want the big compatible version. Don't check any boxes.)
2) Downgraded the firmware from build 5897EX (the one with the Polycomm logo on its local webpage admin tool) to the one that worked the other morning, build 5859EX.
3) Turned on ObiTalk and Provisioning on the device. Turned off Auto Firmware Update.
4) Logged into ObiTalk portal and added my device. Turned off Auto Firmware Update again.
5) Added my Google Voice account. Turned off Auto Firmware Update, again.
6) Made a quick test call to a number that I know didn't work under either XMPP or 5897EX. It worked!
7) Turned off Auto Provisioning, ObiTalk, LUA Script Update, rebooted.
8) Deleted device from ObiTalk dashboard.
9) Edited saved xml file with XML Notepad 2007 in tree mode. (This presumes your GV is on SP1. You're going to delete everything associated with it. Unlike the Admin menu, ITSP and SP are not separate things. It's all together in the backup file.
a) Search for "Google". That will bring you to an object. Delete that object.
b) Click on the next object. Unless it is for voice service 2, delete it.
c) Keep going until you get to voice service 2. You may recognize it by the name you assigned to the voice service. But definitely by the number. Don't delete it -- you're done now.
10) Save the file with a different name than the original backup file. (You know, to be safe and all.)
11) Do the restore
12) Manually put back the passwords for SP2, SP3 and SP4.
That's it! Reboot and all should be right with the world.
Now, I just need a little courage to do this again, this time with the 202 that runs my real telephone world. I'm glad I had the 200 around to play with, but it's not the important one, just a spare plaything.