http://www.mediawiki.org/wiki/Extension:ConfirmEditNot entirely sure if it goes to account creation (which I hope it does/can), but this might work.
Edit: It does!
There are five "triggers" on which CAPTCHAs can be displayed:
'edit' - triggered on every attempted page save
'create' - triggered on page creation
'addurl' - triggered on a page save that would add one or more URLs to the page
'createaccount' - triggered on creation of a new account
'badlogin' - triggered on the next login attempt after a failed one. Requires $wgMainCacheType to be set to something other than CACHE_NONE.
