Not in a pickup truck though....
This is actually pretty common, there are a number of manufacturers that produce truck-bed fuel tanks intended for refuelling other vehicles, you just don't often see them in the city. They're used for transporting fuel to construction sites (it's not like someone is going to drive that skid steer to the nearest gas station to fill up every 8 hours), and also not uncommon for those who enjoy power "sports" (Skidoos, ATVs, etc...).
The whole issue of spillage can also be mitigated with appropriate materials. I own a small (27') sailboat with a diesel engine. When we fill our tank up, you have to pass the fuel nozzle over the water and into the cockpit. When you do this, it's always done with the nozzle pointing up, and wrapped in a sheet of oilsorb. Spilling into the ocean is generally a much worse offence than dripping a bit onto asphalt (which is already a petrochemical product).
Basically, as long as the companies doing this are well regulated, and follow procedures, I don't see the issue. Would I ever use it? No, especially because I doubt they haul diesel around for my TDI, and I'm also enough of a cheapskate that I wouldn't want to pay the premium.
Relatedly, around here at least (Vancouver, BC), I see a number of services that do this with a 2000 gallon tank on the back of a 5 ton truck. They go from construction site to construction site refuelling all the equipment on-site.