This would actually be pretty cool if it was made into an open protocol. Let there be a well-known ID for encrypted paywall content, with children divs that specify which search engine's public key it was encrypted with. Search engine bots would then be able to decrypt the content with their private key.
The downside is that now you're sending duplicated content to clients. Turning it into an event that javascript could fire off would solve this; a bot accesses the site and hooks up to the encrypted-content event. The event returns a list of search engines. If the bot's search engine is in the list, it hits a well-known URL with its name, and gets back the encrypted content it can decrypt with its private key.
The downside is that now you're sending duplicated content to clients. Turning it into an event that javascript could fire off would solve this; a bot accesses the site and hooks up to the encrypted-content event. The event returns a list of search engines. If the bot's search engine is in the list, it hits a well-known URL with its name, and gets back the encrypted content it can decrypt with its private key.
Just spitballing a few ideas here.