I like where you're going with the meta key. I also like the ESC key and all the examples you mentioned. This allows the creation of a very powerful META system. 
You mentioned filtering out R if mid text because it might be destructive. I would prefer not filtering it out and just ensure it's not destructive; it doesn't have to be. Here's the simple use-case:
- I just typed up a paragraph reply and realized I forgot to attach it to the thread. If it's destructive/filtered, I have to select all, then cut, then set the reply, and then paste.
The other main thing I would suggest; which is pretty obvious imo: don't change existing functionality. Just give us another way to do cool things and let users pick their favorite way.
FYI: /away does set your status to away; which is clearly visible on your avatar with the away icon (yellow minus); compared to the online icon (green check mark). Here's the sample:
