I can't think of a reason it would be un-sound. The matte medium is basically gloss medium with a high proportion of silica or other transparent, inert particulates. It ensures that the acrylic dries with a rough, matte surface, and also increases the porosity of the layer. The mechanical and chemical bonds between the acrylic gesso and the matte medium is strong, and the porosity of the matte medium ensures that any oils painted on it will adhere strongly. Now, as to whether they will remain as flexible as the acrylics over time ("fat over lean") odds are they won't over hundreds of years, but working on a panel rather than canvas will help that. Also not caring what happens to your work after hundreds of years.
I'd let a few days in a dry, sunny corner elapse before painting oils over acrylics. The thing you want to avoid is trapping moisture under the oil layer, which can lead to "bloom", cracking, and lack of adhesion. It depends on the thickness of the acrylic layer and atmospheric moisture conditions, of course, but compare the paint to what you know to be really, really dry acrylics and if it's markedly softer or weaker, wait some more.
As an illustrator, I've used this technique a lot. Also oils over gouache, oils over casein, oils over pencil (colored and graphite)etc. etc. Not that I was expecting much in the way of permanence, but all of the above have held up just fine over twenty years or more. Painting on masonite or watercolor board helped, I'm sure. Actually, of all my wacko material experiments, the one that has aged the worst was a pure acrylic technique where I used acrylic fixative or Kamar varnish as a quick way to even out the gloss on successive layers. The solvent-based acrylic trapped moisture underneath and led to some appalling cracking, particularly on the heavily-worked areas.
You say \"sfu-MAY-to, I say \"sfu-MAH-to\", let\'s call the whole thing off.....