From those pictures it looks like the 2 doubles are a fairly good fit in the space in the bracket, but i suspect that fully closed the second arm would hit onto any plug in any socket.
If I'm wrong, my suggestion now would be to replace the 2 double back boxes with 3 singles, otherwise your suggestion of FCU and trailing socket will fit the bill. Double to single adapter plates are available but pricey.
If I'm wrong, my suggestion now would be to replace the 2 double back boxes with 3 singles, otherwise your suggestion of FCU and trailing socket will fit the bill. Double to single adapter plates are available but pricey.