I have now had time to look at the unabridged dictionary, and I assume that Hopkins made up his own words.

"Leafmeal" seems to be based on "piecemeal." (If a project is rendered piecemeal, a forest will be rendered leafmeal.) I was interested to see that the "meal" of piecemeal comes from the AS word for "measure."

"Wan" started out meaning dark, branched out to mean sad or weak ("a wan smile") and now means pale. So whatever Hopkins was thinking of when he said "wanwood", it wasn't cheerful.