OMG I think I've stumbled across what may be the world's first illustration of a green man, actually in this case a green woman. I found it in the Stuttgart Psalter, object of my research for this semester which dates from 820-830 AD at the monastery of St. Germain des Pres.
The Psalmist (135:6-10) speaks of "Him that stretched out the earth above the waters... that made great lights... the sun to rule by day... the moon and stars to rule by night." In this charming picture we see Terra holding wheat and leafy plants in her hands while disgorging grapevines from her mouth. Above her left and right are sun and moon, with a couple stars in the center..
All those modern druid and wiccan types who proudly point to the green man trope as a pre-Christian, Celtic pagan symbol, should note that in this case, the green woman is presented in a Biblical context. Given the priorities of this period it does not seem likely that the Church would be anxious to incorporate pagan iconography into its liturgical repertory -- these psalters were probably used for monastic service or for private devotions. So it's possible that the trope is descended rather from Late Antiquity.
In related matters, I was Googling around for greenman images and came up with this charming photo of a greenman tree person, executed evidently with only a chainsaw:
Paul Sivell carved lots more which are presented at http://thecarvedtree.com/Greenmen.htm. How nice it would be if this kind of stuff could be done in Golden Gate Park, although I do imagine eucalyptus wood is much too soft.