Hey Neil,
I came onto the community to ask this very question! While I was reading your post, a thought came to me so I tried it out and it seemed to work.
This will be a bit manual at first, but create a "Clicked Email" smart list for your email. Then, double click on a number of them and check their activity log for "Clicked Link". Double click that and look for this: http://na-sjn.marketo.com/rs/a10networks/488/Optimizing+ROI+WebEx.ics, or something equivalent. When you find it, copy and paste it into your filter. I don't know why this link doesn't show as an option when you're searching, but it seemed to work for me. Give it a shot and let me know how it goes!