rtmp can be used on squid with a big BUT.
since rtmp is a tcp protocol you must allow a CONNECT and destination
ports to be used through the proxy.
but it's not such a safe and good idea to do so.
since the squid box is a router in your case and you will intercept
the port 80\443 rtmp will not have any trouble if you do use NAT for
outgoing connections since rtmp works on other ports then 80 and 443.
But the routing will be different somehow won't it?
For example, let's assume youtube uses rtmp.
A user connects via VPN, navigates to www.youtube.com, on the VPN server
the 80 request is directed through squid, the video server returns the
80 request and a rtmp request but the rtmp cannot go through squid so
where does it return, just another port on the VPN server? As long as I
leave those rtmp ports open then all is okay?
What if there are 50 clients all using rtmp as the same time, how would
the routing within the 10.8.x.x network happen with squid involved?