Ok, I think I may have cracked the code on getting part of our crew transported to STT.
Dolphin Water taxi makes a few runs a day from Scrub to Red Hook. Cost is $234/pp - includes water taxi, all fees AND taxi to STT. It's more than the ferry but less than a private water taxi for 2. And dropping them at Scrub seems much easier than anywhere else I've seen. From Scrub's website, it looks like Island Time Water taxi does the same. Waiting on rates from them.