Hi Dan,
Since several calendar users have asked for the possibility to add additional fields, it may be a good idea to implement a flexible possibility to add/delete extra fields to events. I will investigate the feasibility of this new feature; as you suggested, maybe something similar to adding/deleting categories.
If this appears to be feasible without too many troubles and we decide to go ahead, then implementing this new option will for certain take quite some time (months). But it's always nice to have something to look forward to
Regards,
Roel