You can check this info out:
Coastal Boating .net, Martha's Vineyard
It does seem to say no overnights in the Pond. The harbormasters hire a lot of kids for the summer.
I haven't stayed in Menemsha for a while, usually better places to stop, but if you go up into the Pond be 100% careful to close your head. If you achor outside the harbor, you are likely to have a rolly sleep except on the calmest nights. Depending on wind direction, consider anchoring on the other side of the Sound for example Tarpaulin Cove.