I had wanted to provide an option to connect to lower bit rate streams on my webcasts that used Multiple Bit Rates (MBR). After a bit of searching I found my answer.
The WMContentBitrate modifier sets the maximum bit rate that can be streamed from a source, in bits per second. So you can simply set the number to be a bit higher then the stream your targeting and direct users where you want them.
There are many more modifiers that can be used with Windows Media Player. For a brief overview check out this article from Microsoft on additional query strings. For a more in depth discussion try Using URL Modifiers.