Thanks pandaperth! I found this after I posted the question.

Looks as though I'll just book dates as far in the future as possible and then amend them when it's possible to do so. Assume that if booked online this will be done through the AA call centre.

Assuming that I can book online, is there any advantage to not doing so? (e.g. to book through a Canadian travel agent instead). I see many references to people doing this, but can't see the advantage except where AA refuses to ticket.