I bought some water mixable oil paints from Holbein (Duo), and Holbein claims that they can be mixed with acrylic paint (in a certain ratio that I don't remember without digging for the info. I think it was 25% acrylic for a faster drying underlayer "Otherwise the old saying water and oil don't mix" applies with traditional oil paint and acrylic being mixed together.

Carl

Oils will adhere to dry acrylics fairly good - and the common canvas primers of today are acrylic gesso (althoug this gesso i matte and will improve the adherence much more than the more glossy film of pure acrylic color straight from the tube - use matte medium in the acrylic paint if you want the oli colors to stick even better). If the paint will still stick to the canvas in a hundred years is likely to be the case - but no one can tell for sure.

The reverse - to paint acrylics over oils - is not that good an idea. You might test this and then scrape the painting with your nail and the acrylic paint will peal off. Althoug acrylic paint has been improved and today it is somewhat common to paint acrylic paint on top of oil (or rather alkyd) paint when painting houses.

Oils (wheter "ordinary" or hydro-) "dries" (to "dry" is not the word a scientist would use) by oxidation. This means that there is a chemical reaction that changes the molecylar structure of the oils. This is a much slower process than "real" drying.
Acrylic (and to my knowledge also alkyd- and damar resins - correct me if I am wrong) dries (now this is the correct scientifical term) by evaporation of water (and turpentine). Hence these paints dries faster. (The difference between watercolor and acrylics is that when the acrylic medium has dried the molecyles will stick wery hard together and can't be desolved with water again while the gum arabic molecyles will easily separate from each other when water again is introduced.)

Oils has a tendency to become very hard and brittle after a long time of drying/oxidation. And if a painting is not painted in a very calculated manner (like the dutch 7-layer method) with "fat" over "lean" then the painting will eventually crackulate when the canvas changes/ages due to changing temperatures and differences in moisture in the air. The museums today has problems with the impressionists paintings.
To try to avoid later cracks one can also do the following things: Paint on new synthetical canvases that does't change as much as linnen or cotton canvas with age or temperature (or at least prime the canvas on both sides with acrylic gesso). Use alkyd medium or even alkyd paints - the syntethic alkyd resin dries to a much more flexible film than linseed oils. If keeping to oils - do not use other whites than those made with coldpressed linseed oils (and not poppy oills, safflower oils - whites made these can be used in the final layers if you want the color to remain more white since linseed oil turnes more yellow with time) in the beginning layers. Also zincwhite (that the impressionists started to use) has a tendency to become brittle and hence most zincwhites are mixed with some titaniumwhite in the factory. Lead white seems to be the least brittle one but has been forbidden in most European countries (and I also think it was forbidden for a while in the USA due to its toxic properties - but the ban was released).

Now, acrylic does it's molecular change and setting through evaporation yes, but I don't think this is entirely so with darmar and alkyd. I'm nearly sure oxidation has a part in that drying as well as evaporation with both resins (still I'm not 100% certain). Certainly it is the case with linseed. Now I have been a professional pictorial Artist on Billboards in which we used commercially prepared Alkyd gloss paints. These resins set up so quickly that I have had incredibly good results with adhesion using acrylic paints over Alkyd dried for a few days (with the alkyd gloss scuffed first, and an automotive type clear coat sealer applied afterwards though). The Billboards were in the direct sun, weather, heat and cold. Conditions no fine art painting would be subject to (excluding murals), so they were a good test of adhesion longevity, when installed for a few years. Still the tough eurethane clear sealer had much to do with the longevity, the acrylics adherance was not so good over time without it.

However, commercial alkyd paints are very different from adding alkyd resin to linseed based oil paint, and different even from artists alkyd paint, which has a great deal of additional linseed in it compared to commercial gallons of alkyd based paints I was using ( they contained none unless added). Which to me, means artist alkyd would need to oxidize for a good deal of time to be considered dry. So as you say in artist paint, no acrylic over oil.

The addition of mat medium to acrylic is an interesting idea, it should add tooth for the oil to bite into. Are you certain Gesso does not have any additional chemical properties to add absorbancy over regular acrylic colors, besides mat medium?
I would guess the formulation of a good primer goes beyond just making the surface mat, but perhaps not. A good primer allows a certain absorbancy for the following paint layer, for a better bond, and working feel. I'm going to ask the tech person at an artist acrylic paint company, I have a contact for. I'll post when I get an answer from him.