Definitions: FED 2.25.1e (at least in my 2004 book
FOUL BALL, FOUL TIP - A foul ball is a batted ball that: while over foul territory, a runner interferes with a defensive player attempting to field a batted ball.
Doesn't say anything about having to be a fair batted ball, so I guess its a foul ball as soon as the interference occurs, and you just call the foul ball. Since there doesn't seem to be any penalty for a runner interfering with a batted ground ball ON foul territory - as opposed to a fly ball OVER foul territory, I guess you have to rule it like any other foul ball. Also, batter would be out if this was BUNT foul on third strike, right?