Don't get me wrong, I have brewed AG and now typically do a PM.
The cost would be more, but for a TON of session brew, I am thinking an extract or PM recipe would make brewing 25 gallons a snap.
Boil 5 gallons with your lb of hops or what have you, add 18 lbs of DME, maybe 3 lbs of sugar. Boil that 5 gallons, cool, pitch a BIG ASS starter, and top up to 25 gallons.
Mash your tons of specialty grains in your usual mash tun, just get the main fermentables from DME.
I am stoked.
(Trash can fermenter in BYO as inspiration)