A few comments folks, from a person with a background in chemistry.
Linseed oil never really dries: This is not actually correct when talking about boiled linseed oil, though it does depend on your definition of "dry". Back in the day, white lead (lead oxide) was added to hot linseed oil to get it to dissolve. The lead acts as a catalyst to cross-link and oxidize the linseed oil. This practice was continued up until the 1970's, when a ban on lead in paint caused a change to cobalt salts in place of the lead oxide. The cobalt salts do the same thing - the catalyze the cross-linking and oxidation of linseed oil. Where this saying comes from is in oil paintings - the linseed oil that is used to thin oil paints and as a carrier for pigments is usually raw linseed oil, and it hardens much, much more slowly than the boiled kind. In some cases, it's decades. Neverthless, I can assure everyone that the linseed oil in one of Van De Meer's paintings has long ago hardened like granite.
About linseed oil under a clear finish - one of the reasons that curled woods like maple "pop" when you oil them is you are replacing the air in the cut cells at the surface of the wood with oil, which has a different refractive index. However, most clear finishes will look just about the same after a few coats as will linseed oil, and the reason is that the finishes either contain oil as a substantial component (alkyd and phenolic varnishes, oil-based polyurethanes, and the like) or have oil-like refractive properties (i.e., shellac and laquer). All of these finishes have a very low surface tension, and will readily replace the air in the cut cells of the wood just as will linseed oil. Since these finishes will do the same thing as linseed oil (replace the air in the cells of the wood), it makes sense that you would not notice a difference after a few coats of these finishes on either a native wood surface or an oiled one.
However, there is a difference between some water-based finishes and the oil-based ones. Depending on the exact finish formulation, the water in them may retain a high fraction of its original surface tension, and it may not replace the air in the cut cells of the wood as an oil would. Moreover, these finishes will actually hydrate the cells of the wood at the surface, which causes them to swell shut. I've actually seen the curl in a piece of curly maple dang near dissappear when I used a water-based polyurethane on it (don't ask - it was a cutomer demand. I poured the rest out on my deck as a sacrificial coat of finish where appearance didn't matter).
Finally, with regards to french polishing, the linseed oil is a necessary accoutrement. Without it, the rubbing pad will not glide over the surface as it should. Instead, it will stick and grab, leaving behind undesirable marks. You can, by the way, use mineral oil for this purpose, though because it will not dray, you must remove it in the "spiriting off" phase.