Belly button retrievers are pretty common on small high performance dighies like International 14''s and 505''s. Gary Hoyt used one with his ''gunmount'' spinacker poles on small Freedoms. They can work reasonably well with a symmetrical spinacker that is retrived forward of the forestay or on a forestay less boat like the Freedom. They have also been used with large America''s Cup boat to help bring the middle of the sail into reach of the ''sewer''.
Most of these retrievers use a sock that the retrieval
line passes through and a "scoop" on the bow forward of the forestay.
Jeff