What about older instruments where it may not be possible to acquire the information needed (source, in particular)?The problem with the regulation isn't the regulation itself, it's compliance and enforcement. I don't have a problem with regulations, I have a problem with onerous burdens imposed by regulations. For musicians, this could easily be handled by having a standardized form, accepted by all relevant government agencies, that declare what woods (+ sourcing) are used in an instrument. Such a form would accompany every new instrument sold (even if the wood is ubiquitous as maple or alder) and would serve as the instrument's passport. Once manufacturers get in the habit of providing that information with instruments on a form that is universally accepted by government agencies, compliance with regulations will be a non-issue for musicians crossing borders.
I do agree that on all newly made instruments there could be such documentation supplied and kept with each instrument. But, if the instrument was purchased at, say, Guitar Center, there is a good likelihood that documentation will be missing come purchase time.