Lol, I don't think you have to worry about DOT dipping your VW's tank if you accidentally spilled your kool-aid in it. If you had a diesel truck, yeah, they might dip your tank if you live in a farming area. Farm trucks are a bigger target because they're often equipped with aux tanks, the fines are based on capacity. If highway patrol thinks they're going to get a big fine out of you, they'll likely ask to search your car first before checking your tank.
If you are paranoid about it, keep your back bumper clean, remove the TDI badges and shut the engine off when pulled over. Change your own fuel filters as well, they will be stained.