No, there's no way to pass arguments that have not been exposed explicitly by the gwt-maven-plugin.
There are still
a few open issues in the roadmap for the gwt-maven-plugin (and there are probably other issues we'd like to add there), but we might make the 2.5.0-rc1 release without waiting for all of them to be resolved.
If you wan't use a SNAPSHOT, then in the mean time you can switch to running the compiler with the exec-maven-plugin.