Excerpts from Elias Norberg's message of 2016-04-13 16:14:12 +0200:
I worked at a company that used the calendar stuff. Unfortunately, I
didn't find a solution to the problem you're describing. I see there
is a program called gcalcli, a command line tool for dealing with
Google calendars. But I'm not sure this would have worked with other
calendar systems.
This probably won't help, but I was able to get the calendar events
to display in sup without mailcap. Attached is my ~/.sup/hooks/mime-decode.rb
file.