Event registration using External Id to match contact.
Mike,  without a customization you can not do this with events/profile.  I am not sure if it can be done using a webform (the webform bypasses the event submission rules) 

You can test this, not on your production site, by setting your unsupervised rule to only consider external_ID and see if it works.  I'm curious.   If that works, then you should be able to use force that webform to use a different rule, but I believe that would also be custom.