they would work, if you have an inline fan for the toilet vent hose. the reason people use the nico vents is so the sun does the air flow thru the composite tank. if you use a normal inline fan you will need to connect it to the batteries in the boat.
that being said, you could put a little 7 amp hour 12 volt battery in the head charged by a small solar panel on deck. the panel could be one of the really small ones ment for keeping a car battery trickled charge, ie about 3 to 7 watts. which would produce about 3.5 amp hours in a day, assuming 6 hours of sun and a 7 watt panel. if the fan draws 2 or 3 watts then it should be fine any more than that and it will not run all night.
edit the nicro fans are fine for splashes, and minor dips