I'm on an IT team in my company and I would like to block youtube to users. I don't want to just deny access to the whole youtube domain, but only to replace the .flv/.mp4 request with the one that I want. That way, if someone tries to watch youtube videos on the network, He'll get a video of why using our expensive bandwidth for pleasure is a no-no.
I thought about using a packet manipulation program and just replace the video ID with something that I want, but I didn't manage to do it right.
I think your best bet is to use a transparent proxy to intercept and redirect requests to your video. It's not for the average Joe but being on an IT team I have faith you can make it work with the existing howtos and tutorials. I did, and I'm just a hobbyist admin/programmer :).
I accomplished it with Squid and iptables.
Edit: What I did was test out this upside down internet setup[0]. Once I'd familiarized myself with that process I tweaked the scripts to accomplish what it was I wanted to accomplish. I think for your purposes all you would need to modify is the redirection script. Hope that helps.
[0] http://www.ex-parrot.com/~pete/upside-down-ternet.html