Logged In: YES 
user_id=469548

Nope, it's not possible to use detectors here. Collision
detection needs to access the form data directly, to get the
@lastactivity data out. I've checked in a simple version
that always rejects edit collisions, but I think it'll be
flexible enough to allow for intelligent merging too (by
overriding EditItemAction.handleCollision).

As a reminder to myself (but someone else can pick it up as
well ;), listing what changes user A actually made would be
nice.