What you could do is create a Usergroup "Search Engines" and give this Usergroup all necessary Permissions.
Then create a Plugin (or File Hack for < 3.5) to make SE member of this Usergroup upon access.
But I think it's not a good idea to force Users to register - you will just end up with tons of inactive accounts.
Users that are interested will register anyway, and those who are not interested are worthless.
Furthermore, this could be considered cloaking and might get punished by Search Engines.
Just my 0,02
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