Could Twitter better support conversations with a special @all address?

There's a couple of problems I've noticed with Twitter conversations:

As the audience of the conversation grows, the number of characters remaining available for the message gets reduced, making it increasingly difficult to say anything.
To make room for the message, recipients are sometimes removed, and then end up missing out on a part of the conversation.

I was thinking that Twitter could solve this with clever use of the special @all account, which Twitter would use to track the participants of a conversation.

Each tweet in a conversation would then show two address — that of the person tweeting, and then the @all address. When you reply-to-all on such messages (so that they appear to go to the speaker and the @all address), Twitter would add your own address to the internally-tracked conversation list.

Recipients on the conversation list would see the tweet from you, with cc to the @all account. Most importantly, everybody on the conversation would receive these tweets in their @replies list (helping to ensure they don't miss anything.)

Open issues and drawbacks:

How do you know who's on the list? (Not a big deal..)
How to unsubscribe from the conversation? (Maybe a bigger deal...)