Perhaps wood putty and some tree wound sealant after it cures?

ordinary cement or mortar work quite well; paint over it with white latex paint if you like. Wood putty has solvents that can damage the tree.

Leave it be. A hole, even if filled with water, isn't a problem.

Anything you might do to "fix" it is old school and will enable rot.

shigo. long weekend. fireplace. wine. read.

shigo performed his work from a standpoint of forestry, but the basic principles of plant response are the same. in short, don't.

You don't have to join the Shigo cult to learn the basics of tree wound healing, although he was the whacky pioneer of our understanding of the process.

I prefer Richard Harris, who digests a wide range of research for the nourishment of his arboricultural vision. Shigo has only a taste for his own cooking and makes theological leaps that defy science, IMO. He is more entertaining than Harris though.

Cement in trees creates a future nightmare for the arborist. I've had to remove dead scaffolds of ancient trees where I have to use a pick to break out the mortar to save my chain saw.

As Strob indicates, current science says "leave wound alone".

Is it better to leave the wound alone than to clean out rot and to drill holes to provide for drainage?

Thanks,

Mark