Given that it takes about as long to pour the shot as it does to texturise the milk, I dont understand how refrigerating the espresso beforehand would be of benefit.
Regarding the busy coffee cart guy, I really think thats just a sloppy, unprofessional way of conducting business. You either serve espresso or you dont.
From my perspective, plunger takes longer to prepare than an espresso. Though if youre intent on the idea, perhaps you could convince your customers by offering this to them freely as a limited time promotion?