Sorry, if this was already wished by someone but I was unable to find such a suggestion: integration of Citavi tasks with Google Calendar/Google Tasks - ideally, with 2-way synch :)
I do not know if you know any German, but there is an experimental Add-On which integrates with Outlook calendar/tasks and also Thunderbird: viewtopic.php?f=116&t=7876&p=25531#p25531 It was not developed by someone from the team (well, at least back then, he was not).
Not exactly what you wanted, but I thought I'd let you know anyway.