I think the only places they could work would be to give the seven 142s from FGW to NXEA to run on their branches for the duties run by single 153s (a Pacer is around thirty metres, so would have more seats than a single unit), allowing the 153s to move to Northern - any extra units for Northern will be used as extra capacity on busy routes, so a 153 (with corridor connections) is arguably more use than a 142 (longer, but with no corridor connection).
Apart from that, I think any routes "down south" are too busy for Pacer operation