I tried to ask for extra miles for diversion once upon deplaning but the FA said "we did not charge you for the extra flying" so we decided to call it even
There was a Murphy Brown episode when she thought her plane was crashing and got her recorder out for her will and said "I expect to get full mileage credit for this flight"
Maybe write to UA saying they should have loaded enough fuel to avoid the diversion and see what they will give you?