I wouldn't recommend that as the brine will penetrate the thawed portions only, until the rest thaws in the brine. Also it may be difficult to remove the neck & gizzards pouch, etc... from the cavity. Having said that, It's only a test bird, right?
I would submerge the "half thawed" bird in ice cold water. Being surrounded by cold liquid will thaw the bird very quickly compared to cold air in the fridge.
Make sure the water stays ice cold for food safety reasons. It should be fully thawed by Thursday, giving you plenty of time to brine it overnight for cooking on Friday.
Jeffro - Lang 60 Patio - Orange Splash-Proof Thermapen - Maverick ET-732 - 2 Weber Kettles