My company has done the same, created an API for all types of 'reward' incentive type programs. Not going into details, but we basically used a logging provider and then have all modules log when appropriate.

Does this require that you modifiy each module to provide the logging code?  Did you come up with an "easy" way to insert your logging calls into a variety of modules? 

(My approach was to use a scheduled task that runs periodically to collect information from the various module database.  The advantage to that approach is that I didn't have to dig into code, and the approach also works for modules for which I don't have the source code.  The disadvantage is that I'm forced to deal with the data provided by the module.  There are things that I'd like to score, but aren't provided in every module.)