FYI, I managed to do this by booting from a remote Jumpstart image – in single user, and then using the following command:
raidctl -c c0t0d0 c0t1d0
Just booting to single user won’t let you do this – you have to be booted from something other than c0t0d0 or c0t1d0, i.e. CD or a jumpstart image.