Generally speaking I don't have anything against such a feature but I would like to suggest some improvements:

• Use the same format like in the other channel (so setPack)
• Make it possible to set the pack temporary (or not) with a parameter (if missing should default to the server's setting in the config)
• Either add a parameter or a separate sub-channel (sendPack) to differentiate between just setting the pack in the storage and actually sending it to the player
• Add a API method on the Bukkit-side for sending this plugin message so that plugins can just use that instead of having to manually send the plugin message and potentially running into issues when the plugin's message channel specification changes

Also please pull the changes that I made to the repository since you forked. That includes an added license-file (GPLv3) which wasn't quite clear before that addition. In order to be sure that you accept the terms of the license and that your code is licensed under that it would be good if that your PR includes that.

I might close this since I totally forgot about it.