In an older version of Matrix I was able to make a modification to make jwplayer display bitrate selections when I provided it more than one version of a particular video. Is such a thing possible with the html5 player in the current Matrix skin, and if so, how can I make it work? Thanks.
No, I'm afraid that's not possible. The skin is now using a simple HTML5 <video> tag, which doesn't have the ability to handle multiple sources.
Edit: More accurately, the <video> tag can handle multiple sources, but it will simply use the first source that's playable by the device visiting the page. There's no mechanism for presenting options to the visitor, or for preferentially choosing a source based on some characteristics of the visiting device.