This isn't a "States rights" issue. It's a commerce issue. You might want to first confirm with MMC that the M3W is Federalized for sale here in the USA, that it's recognized as legal for sale in the USA.
Then, you might want to get a copy of the regulation for registering a motorcycle, specifically, what their definition of one is.
If it is, then you have a case against Maryland for not honoring Federal statute. They cannot make this up as they go along, there needs to be some statutory basis for their denial to register it. You might be able to question the statute with regards to the M3W.
In any event, your dealer should be versed enough in this to have warned you before taking the order. He is the one who'd have to file the papers and get the license plate before delivering it to you. I suppose that's a different issue, though.
