Dirks does not have a platoon split issue (career 104 wRC+ against RHP and 99 wRC+ against LHP). Even in the minors the only time he had a very big platoon split was 2010, otherwise he's been fairly split neutral. So assuming his back isn't FUBAR, they might be able to get away with playing him everyday, although he'd be average at best with the bat (again, assuming his back/hamstrings are not shot).
I think if Pompey starts in CF, and they insist on platooning Dirks, then Pillar should be the 4th OF, not Mayberry. Pillar brings more defensive value and can play an above average CF. If you're tight on money, then spending $2M on Mayberry is not worth it.
The Jays should shy away from platoons. I know it can be a way to maximize value, but too many platoons are not good, and it greatly reduces the effectiveness of the bench. If they do Gose/Pillar, then play Dirks everyday and live with the consequences. If Pompey wins the CF job, then do Dirks/Pillar. Just don't go overboard.