Here's why it won't work:

Customers.

Flat rate works int he automotive world be cause the vast majority of an auto shops customers are not there when the work is being preformed and do not have a firm grasp on how long the actual work takes. Most times they drop off in the morning and pick the car up in the evening or the next day and are charged how ever many hours the job bills and are unaware of the actual time spent on the repair by the tech.

With the sort of on-site service that most of us have to preform we are out at the customer's facility and they, or their employees, see us there preforming the work and know exactly how long we are there. Try to get 8 book hours when your tech was there for 4 and they know it because they signed him out, or worse yet the visitor log tells them. Never mind the fun times the tech is going to have when he has to try and get a work order for 8 hours, plus travel, signed when he was only working on the truck for 4 hours... No thanks!

Shop work it can be used without issue but if your labor rate is based of of straight time you are just kind of ripping off your customer.

Manufactures pay out book time for warranty work to protect themselves from unscrupulous dealers. I have had a chance to look over warranty submissions that a manufacture we work with gets and some of the times dealers try to charge for simple jobs is completely insane!