Found the 'solution', in case anyone else is looking.
The error disappears if I move the declaration of the plugins from application.properties to BuildConfig.groovy. I'm not sure if it's actually "fixed" it or if it caused a clean/reinstall of the tomcat plugin that might have cleared it up, but it removed the error on deploy.
After this I hit another error, with JBoss hibernate libs being older than the ones in Grails 2.1.1 dependencies:
Caused by: java.lang.NoSuchMethodException: org.hibernate.validator.ClassValidator.
(java.lang.Class, java.util.ResourceBundle, org.hibernate.validator.MessageInterpolator,
java.util.Map, org.hibernate.annotations.common.reflection.ReflectionManager)
that was fixed by following the instructions here to tell JBoss to use the app's libs instead of its own.