Well at least it is a local cultivar listed here at Nick Botners collection. So the bulk of apples in the spring scion event in Oregon are supplied from that collection and so there are easy possibilities to negotiate that if you live not too far away.

Apples are not too hard to August bud onto other stocks and if not done soon before trees go into rest then it's gone almost for sure. It sounds like the only life support from the roots are the outer woody tissues.

If you have some root suckering near the trunk, you could also try some inarch grafting using attached growing extension (sion) in a method similar to grafting with detached sion. Use multiples depending on your damage.

Bridge grafting similarly will transfer energy/nutrients past the damaged trunk area. Using detached sion wood... another apple. Waterspouts are a recommended source.

 See Here:   http://www.omafra.gov.on.ca/en.....98-003.htm

and here:   https://www.canr.msu.edu/news/....._for_trees

Some 'heroic?', anyway more advanced grafting techniques you could try nOW.....  always, always, better when pushing energy up...  however, you won't be out anything but your effort.   And if you get takes.... so much the better. I did some top working on a couple of limb stubs around this time three or five years ago... the favorable resultant  takes put me two seasons (6 mos.) ahead of waiting for dormant season grafting times, the way I see it.

The repair will be easier on younger trees where you can find the cambium more easily. Three year old should be easy to get to.

Another inarching.... Rootstock(s), planted near the trunk, using the root trunk (sion) attached could also be used if you had excess to contribute to the cause.