TREE-METAI've been interested in TREE-META since I discovered it while at UC Irvine. We used a version for the UCSD p-System on the Teraks (desktop LSI-11/23). It had a pleasant and compact syntax that was more convenient than the LISP that took it's place for me.
For several years I've been on the lookout for information on TREE-META but haven't been able to find out a great deal. After about the third time listening to The Mother of All Demos I heard them mention that TREE-META was the language used to implement the "special purpose languages", which we call "domain-specific" today.
I've been unable to locate a copy of any version the program or the tech report describing the language, although I've seen hints than some folks have used it fairly recently.
In commemoration of the 40th anniversary of the MoAD (also at Wired), I've created a Wikipedia stub about TREE-META as well as one here for non-WP suitable material. The WP article is very rough, but I'm hoping that others with information will bring it forward and collect it there.