The problem with nail polish or any other sealing coat is that it can seal IN the problem as well as keeping water out.

You're better off to clean the spokes off and use a good paste car wax on them. The wax will last quite a while actually.

In the end it's a poor way to store the bike so a lot of other stuff is going to suffer in various ways as well as the spokes. I know you don't want to hear this about your pride and joy transportation bike but it may be a lot easier to just consider it as a consumable and allow it to erode back to the elements over time and get another bike when it's too far gone to revive any more. For example, if you're worried about a bit of rust on the spokes what about the inside of the frame? A steel framed bike living outdoors in the elements with just a tarp over it is going to rust badly in short order. Yet I don't see you asking about stripping the whole bike down to allow coating the inside with a frame saver concoction.

Consider too that rims used for braking in sloppy weather are a consumable as well. By the time the rust on the spokes is an issue the rims will likely be ground down by the brake pads and the grit they pick up in bad weather. At that point you could "fix" the rusty spokes issue with a new set of wheels that'll have stainless spokes. Or rebuild your old wheels with stainless spokes and new rims.