I would think if you were going to do this you would have a propane tank rack at you shop with 4-6 tanks per mower. So running out would not be an issue and the propane man would just come by your shop once a week perhaps once every 2 weeks and fill up your empty's.
But before id do this I want raw numbers these forklift tanks how much per refill? How many hours do they last on a mower? Whats the cost per hour vs cost per hour of gas or even diesel. If gas prices collapse again and hit 2 dollars is this still a good alternative?