Is it the "button-like elements" part that's throwing us off here? We should probably reword that to "clickable elements, such as a button", or something.
I think Richard has it right though.
The range element should receive key and mouse states just like the handles, except that we're using the ui-widget-header state as its default class instead of ui-state-default.
This can probably remain the case, since it's nice having the range look slightly different than the handles.
I wonder though, when you focus on the range, should it appear that you've focused on both handles and the range, because in reality, that's really what you're doing (focusing on all 3).
It's sorta like a selectable in that way; the range makes is a proxy to control both handles at once.
Perhaps we should mock up some of these situations in a static page to see what feels best?