The tag I'm imagining would just take the YouTube video ID, and then create a full embed of the video in a mini-window, which could be accessed by clicking a second button independent of the normal video search button we already have. For instance...

Here's a nice multi-cam fan video of Nine Inch Nails performing Down In It in Philadelphia on 8/28/08: http://www.youtube.com/watch?v=WMwjvCpXKB8

Now, let's say we want to add that video as a secondary video link to this setlist, with the same location and date: http://www.setlist.fm/setlist/nine-inch-nails/2008/wachovia-spectrum-philadelphia-pa-3d6a907.html

In that case, the line in the setlist would look something like this:

Down In It @Video[WMwjvCpXKB8]

Of course, if this were implemented, then something special might have to be done to deal with videos with multiple songs in them, such as these...

http://www.youtube.com/watch?v=nT6325bmcsQ - Part of the Beatles' rooftop performance
http://www.youtube.com/watch?v=Vk2ftDrvq5E - An entire Deströyer 666 set, or most of it, crammed into one 38-minute video