I do recall having problems outjecting a treemodel from a seam component. I had it working using a managed bean (in faces-config.xml). Now I'm just using a facelets function to convert it (not much use for you I know). I would suggest (a) creating the treemodel in a managed bean and injecting (via configuration in faces-config.xml) the list or (b) creating an extension of the TreeModel componentthat accepts a list and creates a treemodel from it.
Sorry for this being vague - but its been a while since I looked at this.
It looks to me like Trinidad is soon to release an milestone (m1), at that point I think it would be worth getting together a Seam/trinidad example similar to the icesfaces one.