the simplest solution to would probably be just having another thread for chatting
+ the real thread doesn't get cluttered
+ it's easy to link and for everyone to use
+ no additional software/burden
+ it can be on page 7 with thousands of replies, ?last=100
has no drawbacks for a chatting thread
- either somebody has to stream or everyone has to press play at the same time (you can agree on a time beforehand)