I also never see UA allowing this.
And @
jsloan is correct. Marriott counted award nights towards elite status well before the merger with *wood. Based on a Flyertalk search it appears this feature started in 2015. That's long before any proposed merger so I don't think *wood had anything to do with it. They were likely following another brand who did it at the time (*wood, Hilton, etc.).
-RM