Curiously, resistance to oxalic acid is also a question in human health. We are protected by the symbiotic bacteria Oxalobacter formigenes, which normally functions to prevent oxalate kidney stones and is lost with some antibiotic treatments. So my first thought was to find a similar microbe that degrades oxalic acid and teach it to live on chestnut trees.
It's not exactly what you're proposing, but the CHV1 virus attacks C. parasitica and it's being used to combat it in Europe. I'm surprised the article didn't mention it.
Indeed, however Oxalobacter formigenes is not widely present in all human biomes, even those without antibiotic therapy.
I have attempted to garner the interest of major enzyme corporation (Novozymes, for example) in mass producing the oxalate degrading enzyme through fermentation, but no response.
