I don't use a mail reader that supports calendaring, but will often receive emails that have ical invitations in them. They are a pain to try to read in plain text and parse though. Is there a command line tool on Linux to view the calendar requests "prettily"?
Please note that I've not tried any of these so I can't be sure that they will do what you want.
I think that calcurse might do the job.
ical - An X based Calendar Program - OK so needs x-windows, not so good for your requirement
These might do it but not clear if they support iCal:
In addition, there is pal but no sign of iCal support I'm afraid.