Borrowing from an exchange in a similar thread in the Google support forum, as suggested by Op ENkODE, I was able to set my DX to use Voice as the default app when generating a Text Message from Contacts, but if the Messaging App is used, it still goes through the Verizon path. That's good enough for me, as I don't really use Messaging as a pathway to generate Texts.
Here is the method that was presented there, Edited a bit for clarity, with a comment from me in parenthesis:
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Go to Menu > Settings > Applications > Manage Applications > Google Voice > Clear Defaults
This will reset the default messaging client.
(Note: My DX still showed No Defaults selected, so I didn't do anything there, except verify status)
Exit back to the Home Screen
Select Contacts, and long press on a previous contact, then select "Send Text Message". You should see a popup to select your default app. Just choose Google Voice as your new default.
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This seemed to work properly for me. Now, when I select through Contacts, or Contact Search, texts do go to Voice. The only thing I don't quite understand is that when I go back to the Defaults Option selection, it's still greyed out.
After a Power Cycle, conditions remain the same for me.