events

I can think of at least three ways to do this:

-- COM add-in that copies data up to the web calendar when items are added
or changes and periodically syncs deletions from Outlook and any new/changed
items from the web calendar. (Outlook provides no useful events for
deletions.)

-- Exchange client extension that does the same thing. Harder, because it's
Extended MAPI, but potentially more robust because I think you get better
handling of additions and deletions.

-- Separate MAPI store that connects directly to the web calendar and
exposes the web calendar's data in a separate store in Outlook, in other
words, not in the user's normal calendar folders. You'd have to figure out
how you want to persist the data when Outlook is not online.

None of these approaches is particularly easy. Obviously, a lot depends on
the web calendar's programming interface.