I am interested in this topic as well.
I'm not that familiar with TestComplete off the top of my head; as I understand it (and I could be wrong!) TC has some way to export test cases to a version control system (e.g. git). Those commits (or diffs) could then be added to Collaborator just like any other. This does assume that the exported test plans are in some sort of human-readable format; if it is a binary file then obviously a diff view during the review process will not be useful.
(Our QA team does use TestComplete internally on Collaborator as part of their test plan; but I have not used it directly, hence the above is based on relatively minimal knowledge of their product.)
