What do the rims look like under the caps on the Prime's wheels?

You can paint the spindles with boiled linseed oil.

Also, with wheels off, a light application of anti-seize is good on the whole face that contacts the inside of wheel, and in particular at the raised flange that fits into the bore. It minimizes rust "gluing on" the rims (when you attempt to remove them", and maybe further seals the spindles.

I think centre caps are more effective at sealing spindles against water (and salt) then wheel covers? The full covers hide but don't really seal? Could both be used?

With full covers a more rudimentary centre cap might be something to consider too.

 

Armchair engineer says, hemp oil seals best! really, but it turns to plastic over time. Wouldn't anti-seize (I'm thinking copper based, only thing I know) leave staining? I think the plastic contacts from hubcaps touches metal rim and it is not metal to metal? If you lubricate that then you will lose more hubcaps.
Now forming committee for prius hubcap patrol...

 

I would do all of this with the wheel off. Boiled linseed oil dries within 24 hours, provides a durable moisture sealant for the central "spindle". Anti-sieze applied sparingly is common practice, keeps hubs from sticking on, and ensures a nice clean faying surface. I use Permatex silver fwiw.