JBoss doesn't seem to care. It finds the .tld even where it is now (top level). Tomcat, however, is picky. With the .tld in the top level directory, it doesn't find it. It works when I move the .tld into META-INF.
To be clear:
src/plugins/json/src/main/resources/JsonPlugin.tld -> src/plugins/json/src/main/resources/META-INF/JsonPlugin.tld