I was just about to ask the very same question but I'll expand it a bit since there was no answer:
Is it possible to change the split button's theme dynamically? And if not, can you come up with some other neat way to let the user know that something special's going on here? Something to pay attention to.
Much thanks for all participation. All input is appreciated!
If you change the classes applied to the split button using js, you could change the theme dynamically.
So in the js fiddle provided as an example, the button has a class of "ui-btn-up-b". Changing this to "ui-btn-up-a" or "ui-btn-up-c" would change it's appearance to a different swatch. Some simple jQuery.addClass/removeClass would be what you need. Or wright your own class/styles and apply them dynamically to elements.
As far as "something special is going on here". Perhaps add a brighter border to the element your are highlighting? Animation might also help, like having new items slide down to appear like they are being added. Hard to tell you what is best without more description of what you want to highlight and why.