Hey Darren!
That’s a great question. I wanted to say “yes” right away but did double-check and discovered that it is not possible to export custom fields with iCal. the reason has less to do with how The Events Calendar handles the data, and more to do with the iCalendar data format itself, which does not support additional fields. I suppose we could make that happen, but we’d need to hijack an existing iCalendar field and inject our data into it, likely at the expense of something else.
Sorry for the bad news, but does this at least help answer your question? Please let me know.
Cheers!
Geoff