a good question,
i'd say its not possible, but im not sure
(i believe that you can sacrify models every turn, so it wouldnt remain for the battle)
The way i use flaggelants wouldn't vary to much from 6th ed.
using them as flankers to a parent unit (in which case the detachment is in front of the parent unit, causing a turns delay)
or in combination with the rod of power in a cav unit (who doenst like to take initiative and charge a heavy cav unit

)
especially when suporting a cav unit the rank might come in handy!