You can do this by implementing the listTimeObjects and listTimeObjectCategories API calls. Nag (task list) and Whups (ticket tracking) do it this way. If you are using the Git version, you can use TimeObjects, which wraps everything up in a nice class.
Kronolith calls these functions in initialize() (when filling $all_external_calendars) and listEvents(). It searches all APIs for this function, so you can register it with your own prefix.
timeObject is a generic name for "things that can be shown on a calendar", like events, task due dates, ... A timeObjectCategory is a group of timeObjects, which can be enabled or disabled in a group by the user. (In the default layout: using the panel in the upper-right corner of the calendar view, "Calendars".)
Your implementation for listTimeObjectCategories should return an array, with a category id as the key, and the user-visible name as the value.
listTimeObjects is called with the currently selected categories (an array of id's), and a start and end date. These dates might already be Horde_Date objects (to be sure, convert them again: $start = new Horde_Date($start)).
You should return an array of timeObjects that belong to these categories and fall in the specified time range. The keys are event id's, the values are arrays with values for the following keys:
If you register the show API call, this will be used for creating the link to the timeObject detail.