Hi Ian!
I do indeed know about Fluid Film:
http://www.fluid-film.com/testing.html I have not used it myself, but friends have employed it to protect the outside of gun barrels on hunting rifles (it rains and snows here during hunting season) and on the cutting bars of chainsaws. It seemed to work well in each application, but it was visible as a sort of film over the underlying finish. That wouldn't be a problem if it was *inside* tubing, but I think it could change the finish outside (i.e. matte black powdercoat might become glossier).
It does work, but some cautions must be observed. For one, it can attack rubber including some forms of synthetic rubber, and I know of someone who used it to seal the external drive shafts on an industrial water pump against corrosion...and it did seem to be instrumental in damaging the seals.
Also, it is thixotropic and becomes thinner under pressure and thicker when left standing. For this reason, it is best applied in quantity with a *lot* of pressure behind it in order to get adequate film thickness and "spread". An aerosol can do the trick alright for small areas, such as inside tubing.
It is removable with heavy detergents and bio-cleaners. What is not certain is just how long it will provide maximum corrosion protection. It does a very good job short-term, but it appears it must be rubbed in to get the best protection.
Having sealed/treated a number of frame interiors with other products, I can offer on caution: If you apply a sealant in the top portion of the seat tube, it can be difficult to get the seatpost back in after the stuff dries. Even worse is to insert the seatpost before it has fully cured, as it will be more difficult to remove. Don't ask how I know this.
Sealants can also be problematic if they manage to worm their way into the threads of a bottom bracket and can "glue" an eccentric into place with just enough tension to make smooth adjustments a bit difficult until you "break" the film and it can turn freely once again.
There are alternatives. Some people claim outstanding results from linseed oil, but it can also be a mess if it gets into threads and can be a real problem to clean up if it gets outside and then cures. Waxoyl is a very popular spray, as is the sort of liquified aerosol wax used to shoot into car door panels to discourage rust in places where the roads are salted in winter.
Being an iBOB listserv alumnus, I have to mention beeswax. It is not good in its raw state , where it flakes and crumbles, but becomes extremely useful and moldable after it has been kneaded by hand. Kneaded beeswax has worked well for me to seal framebuilding vent holes in chainstays and fork blades, around the base of seatposts at the seat lug clamp, and as a threadlocking compound on threaded quill headsets. It is also terrific as a lubricant for driving long screws into hardwood, as I sometimes do with stair handrail brackets and bathroom grab bars. Another nice thing about is you can always pick or punch it out with a double-ended toothpick, and it is waterproof and replaceable, leaving no trace afterwards if you decide you don't want it. It helps prevent water entry, but the huge limitation is it does nothing to protect the inside of tubing, so it is still wise to use something -- even a grease- or oil-soaked rag run up and down on using a wire coathanger as a handle can help protect raw steel to a degree.
If you try Fluid Film on a bike, I'd love to hear how it goes and a bit about how it seems to be protecting over time. Like you, I really like that it is a natural product and should therefore be somewhat safer. FrameSaver smells something terrible, and I use my charcoal-filtered brazing mask *and* lots of ventilation when applying it. Opening the frame later releases what always smell like fresh fumes to me and is enough to clear a room.
Hope this helps.
All the best,
Dan.