Electronically Actuated DPDT Switch?
There is no good way to do this. Bidirectional semiconductor switches that can handle the motor power are almost as much work and as large as the original motor driver itself. This is because the semiconductor switches that exist fall into two broad groups:


Can conduct current in both directions and interrupt current, but can only block current in one direction
Can block current in both directions but cannot conduct current in both directions and cannot interrupt current (once they start conducting they latch on so current must be interrupted by something else)


That means you need multiple devices just for a single bidirectional switch plus all auxiliary drive circuitry.

Far and away the best approach is to get an ESC that actually allows inherent reversing in software. 


  I could use two mosfets on each ESC


No, you can't. This is wrong for two reasons:


MOSFETs fall into category #1 which means that just to connect or
disconnect a single line with bidirectional currents, you need two back-to-back
MOSFETs just to make a single bidirectional switch.
But you aren't just looking to connect or disconnect a line. You are looking to connect/disconnect a line from one destination and disconnect/connect it to another destination. That means you need two bidirectional switches (4 MOSFETs) to swap one line between two lines. And you want to do swap two lines which means you need 8 MOSFETs. That's more MOSFETs than are actually in your ESC (which needs 6; if you have more than that then they are just all in parallel in 6 different groups for higher current capability).