Its anecdotal, of course, but the "crustiness" I recently found under one of my front wings might provide some pointers on rust protection.
When I first got the car, 5 years ago now (Jings!) I gave it a fairly thorough spray down with diesel fuel and veg/motor oil. I took the front wheel arch liners off to spray behind them.
Since the car has bolt-on wings, I thought it'd be satisfying to take them off and get into the inner-wing rust traps.
Starting with the passenger side, I couldn't get the last bolt off and eventually gave up, but in the interim the wing had apparently distorted. I had a hell of a job getting the bolts back in, and had to do without one of them. There was a bit of a gap, and the door wouldn't shut.
I did A LOT of messing around with struts inside the rear of the wing, pulled into position by a wire Spanish windlass arrangement (still in place) , before I could get the door to shut OK
So far its just another "If it ain't broke..." story, but there's a sequel.
Over the years I'd occaisionally dribble oil on the wing join. Not planned, just ad hoc when convenient. The ill-fitting wing (and especially the empty bolt-hole) may have given better access, or it might have just tempted me to favour that side, since you could see the oil being drawn in there by capilliary attraction.
When I took the wheel arch liners off a couple of weeks ago, there was some fine oily sand/silt on the flat inner wing passenger side rust trap, but absolutely no rust that I could see, in sharp contrast to the other side.
Easy access and ventilation is good. Getting rid of carpets (which gives access to the inner sill holes) and drilling injection holes through floors into box sections might be on the adjenda, though of course they wont suit everyone.
My front door bottoms have mostly rotted out. I think filling and patching them might hasten decay so I've left them open.
Oil is nicely penetrative but asking it to protect for 5 years might be a stretch. Would Waxoyl have done a better job here? Dunno.
I think the arch-liners do have a protective function, but they also limit access, so I'm undecided whether they are a net benefit. I'd probably have spotted this earlier if they hadn't been there, but it might have got bad faster.