I did some further checking.
Our developers have chosen no to integrate Collaborate app into the Android Contacts - the work needed wasn't deemed the best use of time and effort for the number of people who would probably actually use it (since most folks just call other Collaborate users rather than Contacts).
Setting Collaborate as the default calling app only affects calls made from click-to-dial on web pages or other places you would get a pop-up asking which app to use. Since that pop-up asks you to choose between Collaborate and Contacts (and whatever other calling app you have installed), Collaborate cannot be set to be the calling app used by Contacts (it's either-or).