Starting with 1.6.0 you can use plugins.fireHook('filter:meta.getMetaTags', { req: req, data: data, defaultTags: defaultTags }, next);
If you want to do it on 1.5.2 you can try the below using the the hook filter:topic.build.
myPluign.onTopicBuild = function (hookData, callback) {
hookData.res.locals.metaTags.push({ property: 'tags', content: 'abc, def' });
callback(null, hookData);
};