Get the older guys to coach and I mean the older guys.
Have a little program with a bio of each alumni printed on them. Get them out several months ahead of time in order to get them back.
Are they going to play each other or your current team?
Reason I ask is
If they play each other check on insurance coverage in case someone gets hurt. See if the school covers them, get a cheap one or sign some sort of waiver.
If they play your current team check with your state association to see if this is allowed. I know some states only sanction contests (regular and scrimmage games) against other high school teams. Some say you can't play these games during the season - only in preseason.
Basically cover all bases so nothing jumps up and bites you in the rearend.
Another thing you may want to look at doing is take a home game per week and recognize a decade. Have them come out and throw a first pitch as a group to your players lined across the IF. Program with a bio and introduce them individually to come out to throw the first pitch.
First week of the season recognize the players who graduated in the 1960's
Second week of the season recognize those from the 70's
Third week is the 80's
Fourth week is the 90's.
Or you can do the last 4 weeks of the season. Only problem is rain and canceling / moving the game.
This is a great idea you got.
When life hands you gators - make Gatorade