Caps - yuck - that reminds me of the fake nuts on plastic wheel trims
Or maybe you could get round black plastic caps as used on the wheelnuts of various British Leyland cars in the early 80's - Allegro, Princess, etc. I never like the idea of covering over rust, it's just not right
Though, I guess you could grease the bolt head under the cap.
Why not just buy new bolts?
I went to my favourite tyre shop and bought a set of 20 bolts with a pleasant chrome-plated finish (actually they are slightly less than chrome - more a satin - you can see in the photo and decide for yourself). Cost $5 each (2.50 pounds).
Bolt spec is 17mm hex - M12 - 1.25mm-pitch thread (be careful not to get BMW-spec bolts, as those are 1.50mm thread pitch).
Unfortunately there was only one length available, and it was 8mm too long, so I did have to hacksaw every bolt down to the right length and carefully round the ends on my grinding wheel
The hacksawing is a good workout (buy a few new blades) and you get pretty fast by the 20th bolt, I have it down to 4 minutes each
As a slightly-cheaper alternative (about $4 - 2 pounds each), you can have the original bolts chromed. That involves the shop buffing out the corrosion, copper-plating, nickel-plating, and chrome plating. When I've had this done, I've found they usually like to cut down the hex first to ensure it doesn't end up too large. But I don't like them doing that, because then the sides might end up not-parallel. There is always the risk of the chrome cracking later.