The plastic used needs to be oil and UV resistant.
I wouldn't bother too much about oil resistance, as there are already plastic parts inside Shimano and Rohloff hubs, and even in Sturmey-Archer hubs from the 1950's.
Because of this, if using oil lubrication inside Shimano hubs (and Rohloff) you should be careful about the composition of the oil you use. My own choice is a synthetic gear oil, other people have used hydraulic fluid, apparently with no problems.
UV resistance might possibly be a problem on a nylon screw outside the hub, but the poster of the link I provided didn't seem to have any issues. If they do degrade under tropical sun, they are cheap enough to be replaced every few years. And Sturmey-Archer used plastic oiler caps on their 3 and 5 speed oil-lubricated hubs in the early 1980's, I still have some of these, no problems after about 40 years of use.
