I learned the hard way about 7 years ago, don't try to paint another colour over metallic paint as it becomes a horrible gluggy mess. I was using enamels at the time. Does this apply to acrylics as well?

I wanted to use the crystallised salt method to weather seaplanes, flying boats and Japanese aircraft with their dreadful paintwork. I can't weather a model, worth a damn, any other way.

If I applied a base coat of metallic acryllic first, coated it with Future/Klear, let it dry, sprinkled the salt over water droplets on the model, and then spray the top coat(s) after the salt crystallised, would that work? Or is that one step too many using acrylics?

It can't hurt at all. I've brush painted acryllics over enamels in the past. You have to make sure the enamel is completely cured first. A coat of Future in between would be just another protection.

The black nose stripe and red nosecone in the picture above are both acrylics painted over a metallic acrylic. I didn't put down any gloss varnish over the metallic before I painted over it and all this was done by brush.

Generally, given the proper time to dry, you shouldn't have any problems painting acrylics over one another. I can't say how an enamel and acrylics paint coat would interact as though.

Your method for salt weathering sounds like it should work but it might be an idea to test it out on some old kit or cardboard just to be sure.

I haven't had a problem with enamels, acrylics or lacquer based paints. if you see Here I explained how to do the salt weathering technique over Alclad II on a recent build.