So, I can't get the key from a black 2 game to unlock the 3rd regi-pokemon so I was thinking of picking one up from one of my ruby/saph/Emrd games and wondered if it would still act the same to unlock regigigas
No. It wouldn't work.
It would only work if you get it in your game.
Source: Personal Experience
Hope I helped!
Regigigas didnt exist in gen3 games.So you have trade the Pokemon from a friend.
You need to trade a regi, regirock is useless.