A custom-made toptube mounted stem is certainly one option for you. I believe R+E Cycles or Cycles Chinook would make one. Assuming the top tube is round, it should be fairly straightforward.

The lowest stack height stems use 36-40mm of seatpost, but are generally at a much shallower angle than 35 degree stoker stems. You may also gain some stack height with a lower profile sadde and/or lower profile captain seatpost. If you haven't already done it, make sure your eccentric bottom bracket is "up" rather than "down".

I'd recommend going over to the introductions section of bikeforums and welcoming new members. Once you have 10 posts, you can add some pictures to aid this discussion.