I'm finding it useful to attack the TEI Guidelines with XQuery once in a while, to dig out and format stuff for review and discussion. However, you can't just load the P5 source into eXist, because you'll have problems with entities and have to mess with DTD catalogues. Instead, you can just expand the guidelines-en.xml file to pull in all the content referenced through entities, like this:

.../P5/Source$ xmllint --noent guidelines-en.xml --output guidelines-expanded-en.xml

That gives you a 6MB file you can upload into eXist and query easily.