Personally, I believe the whole roll-off concept is out-dated, and does not achieve the outcome we all desire - putting together a shield team consisting of the most worthy bowlers.
I suggest that rather than a single weekend rolloff, we should consider a number of nominated events/tournaments through the year, in which points can be earned by those who wish to nominate to the shield team. There would be a fee to nominate in each event (thereby spreading the cost over the year). The issue of injury would be eliminated, as the points could be made up in subsequent events. It would also demonstrate the performance consistency of each bowler, across a number of conditions/formats/times of day that could be used in consideration if people still wanted a 5 + 2 approach. The selection criteria could be set clearly, and be measurable, removing any arguments.
If a roll-off was still desired, another approach would be to hold it over two weekends, 24 games with the top 18 games counting. This would achieve similar outcomes as above (injury, consistency, multiple centres), and 7 past the post would work.
As a coach, I favour the first (with selection) because past performance is a better indicator of potential than a single event can demonstrate. This provides the opportunity to put together the team with the most potential to perform.
Coach from Hell:

nfire: