Yet another suggestion:
1) Add rules integration to Ubercart, i.e. there will be an include file similar to CA's include file.
2) Add settings that a user can decide where the events are sent (default to CA).

Like this a newbie user doesn't need to install Rules, but if installed she can set it to Rules.

Disadvantage
- more code to maintain

Advantage
- Rules integration
- After a while community will give feedback and we can drop one of the solutions.
- Power users can create more complex rules.