As far as etherpads go, I don't see any reason to get overly complicated with it. Having an on-site tool to be able to collaborate together would be an awesome improvement all in itself, and I hardly think anybody would be against the idea. A PM system clone, converted to an etherpad does sound like a good way to do it, though I have negative experience in coding so I wouldn't quite know.

However I think you should just take the 'New Reply' box, clone it into a module, and add the ability for others to join like a convo system. The 'new reply' box has everything you need, and can be controlled by whomever started the 'conversation'. Add a little window for present users, an add/remove user button, and maybe a small chat box.



As for the dice, the biggest issue is the ability to people to discretely re-roll until they get the number they want. I would recommend maybe another, optional?, tab 'Dice Logs' will a roll button, that records the user's name, roll number, and an optional box for them to write in the action they are rolling for.