A recrystallization should be just what you need. I imagine most of what is left is not DMT n-oxide but rather plant oils or the other unidentified alkaloid yellow oils. I don't think the oxide is usually present in very large amounts without specifically trying to make it by exposure to air after extraction.
This is a nice enough guide.
I guess if your bark is very old though it may have turned a significant amount into DMT N-Oxide. According to the DMT Nexus Wiki entry DMT N-Oxide isn't even soluble in naphtha, so it shouldn't even have been extracted in your pulls. But failing that
this is a post detailing how to convert the N-Oxide into freebase DMT using zinc dust. Might be a little complex and you have to make sure to filter it all out very well.
But if you are applying to leaves, then the N-Oxide isn't even particularly undesirable. The effects are similar, a little bit longer in duration and slower in come up. Many people even prefer the oxidized version to regular DMT freebase. So if I were you if I would do a recrystallization to get out all the oils and then evaporate it on some leaves. The N-Oxide should be cleaned out with a recrystallization I would think as well considering that it is supposed to be insoluble in naphtha and heptane. I can't find any reference to the N-Oxide forming a salt so I don't believe it's solubility changes with the addition of acids or bases.