This would work for non-Newtonian speed bumps as well, though the first time a plow driver doesn't see the sign, and there's always going to be one who doesn't see the sign, you end up wiping out the speed bump instead of just dinging it.
Maybe you could have protective ridges at the side of the road which would act as hard stops for the plough while staying out of the regular path of traffic?
I'd also be concerned about the effect that widely changing temperatures would have on the viscosity and hence critical speeds. It'd suck to install one in summer, get used to driving over it at 40km/h, then during the first cold snap find out it'd turned as hard as rock.
The difference though would be minimal at the recommended speeds which is why it probably is not a factor. If they were designed to keep speeds at 40mph then it probably would be.
Usually the thin small ones are removed in the season, and the asphalt ones have signs. This is the same reason the speed bands/rumble strips are concave not bumps.
City plows also have shock absorbers systems that let them ride the bump, but you do get ice pockets after the obstacle.
