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18-year-old actor Tom Moorcroft will play Regulus Black. "Exclusive interview" April 6, 2008. (He never read HBP.)

My problem: I don't remember Reggie doing anything at all until Kreacher tells the story in DH.

Tom Moorcroft Discusses his role as Regulus Black in "Half-Blood Prince" - The Leaky Cauldron
He does confirm he worked only one day, "...it took about 2-3 hours to complete the shot."

That suggests to me that it's just a cameo, introducing the character for later.