Keeping Timeline / Log of Unit Group Assignment

We have had many clients recently who need to assign some units a certain supervisor who is responsible for monitoring a group of assets for a certain period of time until a job or project is completed.  We advise them to use unit groups to do this.  The problem they encounter is that they need to keep track of  the time period that the units were assigned to that supervisor because later on those same units will get reassigned to a different supervisor for a different job/project.  They need to know for example from June 1st to June 30th, Unit 1, 2, 3, and 4 were assigned to the supervisor Joe Schmoe for Project ABC, then from July 1st to July 31st units 1, 2, and 3, got assigned to the supervisor Rick Slick for Project DEF. 

I'm wondering if any other clients have come across this situation and if so, how did they handle it.  If not, do you have any ideas how this could be accomplished?