The event that triggers the script in the object that is to be attached is when it rezzed. The event that triggers it's rezzing is when the owner sits on the seat prim. This works very well as 'owner only', which of course, is not quite what I want. Because the prim that is to be attached is not linked when rezzed I can't influence it's script with the CHANGED & CHANGED LINK event, which would be able to detach the UUID of AvatarOnSitTarget, as you said. That could be used it it is possible to offer attachment direct from inventory. At the moment I just have an object that gets rezzed and offered for attachment when the owner sits, rather than anyone. I though about using a close range sensor OnRez, but it seems clumsy given the new streamlined nature of AttachToAvatarTemp.