havent' heard that one before... unless things have changed in the past few years, private planes are required to file a advanced notice of arrival with customs but you do that before coming back in, not before leaving the US. pretty easy now with modern communications but back when this came into effect (mid 80s, at the peak of the drug war) it was nearly impossible to call US customs from teh islands. major headache.
FAA handles flight plans, but USCG doens't handle float plans... i doubt that's going to change any time soon. foreign flag vessels of certain tonnage have to advise customs/USCG before coming in, i doubt that's going to be extended to US flag.
LBO is a time saver if you go back and forth with the same crew, otherwise it's useless since your pax still have to go to immigration in person...
I can't imagine customs requiring advanced notice and float plan while still allowing you to clear by phone, doens't make any sense.