If they're taking a long time to get back to you, try tracking down the authors of books with similar subject matter to see if they have any advice (like I just went and did about 10 minutes ago). If they can't answer your question directly, they should be able to suggest books. I've done this a couple of times and they've always been very kind and helpful, although the trouble with book suggestions is that they aren't always in my library (I live over 1000 km from the sea, more's the pity, so naval and maritime history is not exactly a priority. The university I'm at has some really great history courses, but no naval history. Did just turn in a paper on the Royal Navy for a British History course where we were allowed to pick our own topics, though.)
This still seems odd to me. Did you specify in your email to NMM that this is a merchant brig? Or is it on some type of government service (hydrography, charting, diplomacy) that might justify it being HM Hired Vessel or some other such distinction?