"no worries"; and it won't be good advice until it works
Seriously, though - I think that jumping off from the BVI is going to be tougher than from other places. The BVI, while beautiful, is more of a charter's hangout than one for cruisers.
I've met people on the Marigot docks asking every arriving dinghy if they are heading south and if they had space available and those people were never there more than 2 days running, so I would think that the chances are pretty good from there. I forgot to mention the bulletin board (and bar) at Barnacles - it is a hangout for sailors and liveaboards alike for happy hour, although the crowds thin appreciably after the happy-hour-is-over-bell is rung and drinks are no longer $1 per brew. Also the bulletin boards at lagoonie's, the bar at the Dutch Bridge and at Jimbo's when on St. Martin.
I might take you up on the Martinique offer, it is always so much better to get a non-standard tourist view of places. Despite being a hun (and speaking Quebecois instead of real French), I like the French islands!
If you are around Nanny Cay then do visit Captn Mulligans and get some great burgers and mention at the bar that you are seeking passage east and south and you might be surprised how quickly word gets around! That approach might work at Peg Legs restaurant or at the downstairs bar as well, but that end of the docks usually gets a lot of charterers but one never knows what might come available. I know that I've been asked a couple of times if I was heading out and wanted guests/crew at times where I was pretty certain I hadn't mentioned my travel plans so it shows that word-of-mouth travels pretty fast.