Very difficult. It's possible under Campbell considering she was leading at the beginning. Still, you have to avoid all those stumbles, the face ad, the poor debates. It's tough. If you have Charest as leader you may not have the Campbellmania that she had at the beginning, but in the long term the PCs would be more competitive in Quebec against the Grits and the Bloc. Charest would most likely have defeated Chretien in the debates.
But having Charest damages the Tories in the West and possibly opens the door for further Reform gains. 1993 was going to be a bad year whoever was leader. If you have Campbell you lose Quebec, but if you have Charest you could lose the West.
In my opinion Charest had the better chance of keeping Official Opposition. Returning to power is another thing entirely since he'd have to kill Manning and Duceppe. Official party status and financial support helps but doesn't guarantee that. Still if Charest was Leader of the Opposition during the 1994 Referendum he might gain some momentum, some star status, but it's hard to say.