I would use a state machine. Use one state for the off time, then turn on all of the signals, then another state for a 1ms delay. Sort the pulse width of the servo signals from shortest to longest. Then subtract to determine the time between the falling edges and the time from the shortest pulse's falling edge to 1ms. Use the state machine to step through the falling edge delay times, turning off the appropriate signal as the state changes. Some means of dealing with edges that are too close together may be necessary, depending on desired resolution and MIPS.

Cory Spackman