Is there anything in Bolt12 that can allow offline generation of offers, like you can with on bolt11 by setting the payment_secret to a HMAC + ID used to derive a preimage/payment_hash, and possibly payment_metadata that is sent in the onion?

Yes exactly. It would be great to have a library that does this, so that people who don't want to run a cashu mint can still have the cashu UX natively on their BOLT12 lightning node.