I know hot water will migrate up not down on its own in a hot water loop but how much if any resistance will it add to a circulator that is trying to circulate water from overhead down and back up from three separate 6' sections of fin-tube baseboard? The height from the baseboard to the ceiling is about 7'.
I assume you have a 2 pipe system? Water in a typical 2 pipe closed system is unaffected by gravity. The additional piping will add some head loss to your system. Is this a separate zone or are you extending an existing?