OK nevermind, I've found the solution:

- go to application manager
- main menu -> tools -> application catalogue
- for the collabora repository, remove everything in the "Components" line, and replace with "user"
- click OK: the packages list will refresh and rtcomm-beta will be updateable...

Edit: it should also be possible to replace chinook with diablo (if you are using diablo, and you have specified a specific distribution) in the distribution line...apparently packages for Diablo are available (in the end they are the same, but I prefer not having too much dependencies on old stuff found in chinook...)