Is there an autocmd event that fires after saving the undo file?

I'm trying to write a plugin that will set the 'hidden' attribute in
Windows for:
swap files
undo files
backup files
viminfo file

I can update swap files on CursorHold and file write, I can update the
viminfo file on VimLeave, and the backup file on BufWritePost. But I
can't figure out how to update the undo file, because when BufWritePost
fires the undo file has not yet been created (or is locked);
filewritable() and filereadable() return 0 during this autocmd event.

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