According to the playing conditions, I think it is only "non-qualified" players who have to have played in the group stages (it is silent on qualified players). So Ballance would be OK:Aug 17, 2017 - North Group at Headingley, Leeds (night)
YORKSHIRE from: Tim Bresnan (capt|), Karl Carver, Matthew Fisher, Tom Kohler-Cadmore, Jack Leaning, Adam Lyth, Shaun Marsh, Steven Patterson, Liam Plunkett, Azeem Rafiq, Adil Rashid, Sarfaraz Ahmed (wkt), David Willey
In order to be eligible to represent a County in a match in the Finals Stage of the NatWest T20 Blast and/or the Finals Stage of the
Royal London One-Day Cup an Unqualified Cricketer must have been:
(a) registered by that County and played in at least one match for that County in the Group Stages of the relevant Competition;
(b) registered by that County and named in that County’s squad for an abandoned match in the Group Stages of the relevant
(c) registered by that County for a match in the Group Stages of the relevant Competition and obtained a medical certificate to
confirm that he was unfit to play in that match.