I just realised that it is possible to send face-to-face invitations to user's primary Outlook calendar (rather than an internet calendar). I thought this might be useful information for other people, so I'm posting it here.
What I did was:
Open the mod\facetoface\lib.php file
Change all references to iCAL to vCAL (watch out for a few words that have ical in them - like practical).
Change all references to .ics to .vcs.
It seems to be working really well so far. Fingers crossed I don't run into any problems!