Have YSF Server calculate if an aircraft is leading a formation (or easier would be a default leader tag in the .dat, or a radio command inflight.)
From there, the client of a formed pilot calculates the average position off the aircraft consistantly.
This position is shared with the server, the server informs the clients... so everyone sees a formation as being 18inch wingtip to canopy.
The code architecture exists for this (auto pilot formations) just needs to be adjusted a little. Lets set a minimum range for this to take effect to be say 20 meters or so. any very SUDDEN change in attitude or controls from the lead breaks the cycle and the pilots fly by normal positions.

Just an idea though.