This weekend a started to play aorund with new new RAP API introduced in 2.0. It really looks promising.
I started off with the CKEditor Example and created a combo-replacement based on Select2.js.
By the way I had more a fight with the select2 api (that I didn't know) than with the rap api thats quite straightforward. I could answer nearly all of my questions from the source.
It supports markup and (what I always miss on the standard combo) it has "clear selection".
When you create it with SWT.MULTI it supports the cute Multiselect feature of select2.
Here's the example for those who want to try: