My understanding is that if there's any question about the species of wood Customs is allowed to confiscate the instrument until a determination can be made. There is a test involving fluorescence of swab using different solvents that is supposed to be able to distinguish BRW from other woods with similar grain structure. Not every border post has the wherewithal or expertise to do it, which is one reason all shipments of rosewood have to go through designated ports.
Also, how exactly do you prove to Customs that you won't sell the thing once you've taken it out of the country? I suspect that's one of the functions of the 'instrument passport': if you leave the country with it, and don't bring it back in you could be in trouble.
Keep in mind the old adage that 'when you're a hammer, everything looks like a nail'. To Customs agents you are a potential smuggler, and that will be the default assumption. It's up to you to show than that you're not. You're not exactly 'innocent until proven guilty' in a case like this. Rather than trying to skate through by blinking your innocent baby blues it's better to find out what you can about the regs and at least make an effort to comply. You do get some credit for that, even if you haven't done everything exactly right. Then
you blink your innocent baby blues....