I wonder if this would be better implemented as a filter, so that it can work with any CopySpec based task (such as the archive tasks, or a future ftp/scp/webdav task)?

I'm completely open to that idea. Wouldn't a CopySpecVisitor apply to all types of copy task as well, though? Regardless, we should do it in whatever is the standard way for Gradle so users have a consistent API to work with. I'm much less fussed what that standard approach is

My solution

war.dependsOn 'native2ascii'

task native2ascii << {

    ant.delete() {
        fileset(dir: "${processResources.destinationDir}") {
            include(name: 'bundle*.properties')
        }
    }
    ant.native2ascii(src: 'src/main/resources/',
            dest: "${processResources.destinationDir}",
            includes: '**/bundle*.properties',
            encoding: 'UTF-8')
}

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