To my knowledge it has never been tested much less approved. And as you have pointed out grains not only affect the pH, increase the possible contamination, but would also require different and longer processing if canned.

IMO it is another example of the many unapproved, untested, and potentially unsafe canning recipes out on the net. But to be fair, most of the recipes I checked calling for its use stipulate they are for fridge storage only and even then most recommend 2 weeks only. So if someone makes it and thinks they can process it for shelf storage or if someone makes up their own recipe using it and "claim" is it safe to can, that is their mistake.

The only approved use for grains in canning is in fermentation.

Dave