This seems adequate as things stand - I've done a site (http://www.breconfans.org.uk/enter) where users have to register before they can purchase, and once they have done so there is a T&Cs statement on the store page "Declaration: entering the races on-line constitutes acceptance that you participate in these events at your own risk and accept that the organisers shall not be liable for any accident, injury or loss as a consequence of your participation. You agree to abide by UKA rules and obey marshals’ instructions." This gives all the assurance I need - it is (I think - not tested in court!) the equivalent of the signed form they would otherwise complete on the day.

This is a fantastic feature that has worked really well for me, though I can see it would fall short for more complex online shops.