As mentioned before, setting up mod_proxy is probably one of the easiest ways to go because all you have to do is add a configuration to apache, which you already have.
If you wanted to do it with mod_rewrite for a particular URL under the same hostname...Like if you have www.yourhost.com as apache and everything under /video coming from the ffserver you could do:
RewriteRule ^/video/$1 http://127.0.0.1:8900/$1 [P,L]
ProxyPassReverse / http://127.0.0.1:8900
If you want to keep it on the same hardware, but don't care if they resolve to the same name, you could also setup a virtual network interface on a different IP address so that eth0 is apache running on 192.168.1.1 and ffserver running on eth0:1 with an ip address of 192.168.1.2. Then they can each have their own name in DNS, and since they are different interfaces you can have 2 services running on port 80 on the same box.