Per discussions with (and much help from) Hans, Adam and Jason I developed a Gradle plugin for Antlr grammar processing. The code is currently just part of my project's buildSrc: http://fisheye.jboss.org/browse/Hibernate/core/branches/gradle/buildSrc
The only pieces really left to do are:
1) to account for glibs (or supergrammars). The problem here is that the glibs would need to be "linked" to a specific input grammar.
2) perhaps add a convention for globally setting some of the properties (the trace options come to mind).
Anyway, let me know if there is anything I can do to help clean this up.