Never done it in 57 years of owning turbocharged machines, and I often hauled them several hundred kilometres at 100kmh (62mph) with open exhausts.
Never had a turbo failure I could definitively trace back to spinning without lubrication whilst zipping down the highway.
All the turbos I've ever experienced failure with, either had shattered valve heads go through them or they simply wore out and started leaking oil into the inlet side.
Compressor/turbine wheels only turn at very low speed from air passing the exhaust pipe, and residual oil in the bearings is adequate for lubrication under those conditions.
I think perhaps the earliest turbochargers (mid-1950's) might have had some problems in this area, because the earlier turbochargers were bigger, the compressers/turbines/shaft were bigger accordingly, and they more than likely spun with more speed due to inertia - and perhaps the lube systems for them contained inadequate residual oil.
Also, none of the early tractor engines were muffled, a muffler slows a lot of air movement.
As well as all that, tractors of the 50's used oil bath air cleaners, so air movement though a stopped engine was easier with an oil bath air cleaner, than with dry-type air cleaner elements.
The other factor is the fancy curved-end exhausts, such as Allis-Chalmers used - if these faced forward, they would act like an air scoop. I've only ever had straight pipe exhausts on my machines, and many had rain caps.
