You could use the Groovy Plugin (not groovy post-build) and execute a script as first action of your build (you need to use a system script in order to access Jenkins' classes.
Note however, that the mananger
in groovy post build plugin is a shortcut, so you would need something like:
build.getActions().add(GroovyPostbuildAction.createShortText(text));
Update:
Since the above construct does not quite work, try:
def action = new org.jvnet.hudson.plugins.groovypostbuild.GroovyPostbuildAction(null, text);
action.color = "grey";
build.getActions().add(action);
Alternativly, you could just set the description:
build.description = text