Is it appropriate to send the abstract to the editor and ask him if my paper is a good fit with the journal?
Sure.  The benefit is really not to get wrapped up with a long review cycle for something that doesn't make it in anyways when you could have gone elsewhere.  

That said, I think the very top journals (in science at least) like Science, Nature, PRL are pretty up front about weeding out stuff that is not notable and also committed to some speed in publishing.  So you might just get a fast answer on notability, regardless by running through the regular submissions process.  "We just don't publish cuprates that don't superconduct" was an actual editor communication from one of those magazines.