A riding partner once had a freewheel cover plate back off during a ride on an isolated road. We sat in a cold pouring rain, picking as many bearings as we could find, out of the cracks in the asphalt. After what seemed like an eternity, we eventually reassembled it and managed to complete the ride. After that, overhauling a freewheel in the luxury of my workshop, with all the proper tools at my disposal, is a piece of cake (but I still avoid cake whenever possible).
On the customer side, I stopped servicing freewheels years ago. The price of replacement is far cheaper than servicing and in the process you can provide the customer with a Hyperglide compatible unit that will shift better. The only drawback is the more limited ratio selection. Still, that's not an issue for the vast majority of customers and even if it is a concern, they invariably opt for the less expensive solution when they learn the relative costs. I can't remember the last time that I serviced a freewheel for a customer. (98)