Richard,
Your concerns are VERY valid. We've used WebEx to host webinars and have allowed registrants access to the recordings of the videos via YouTube and the one major complaint (before we were using Marketo) was that we couldn't track who was watching the video after the fact. We've been excited, as new users, to be able to track who's watching the videos, not only webinar registrants but for any web activity.
Why can't you create an on-demand environment from within Marketo? We actually have our Marketo "served" videos on our YouTube account, but people access them through a Marketo landing page with a form so we can collect name and email information...actually we are going to implement the first one today, but we've tested it and our support help from Marketo has signed off on our design.
Rodney
I'm not sure what your budget is, but we were having some similar concerns and found out about Vidyard. They integrate really well with Marketo, and were orginally backed by Salesforce and YouTube. We were totally sold on their reporting, analytics, and ability to integrate with Marketo.
That being said they are on the pricy side, but they seem to be really willing to work with budgets.