Is it possible to encrypt a message to a public key and have it be only decrypt-able by the person that owns that bitcoin address?
Yes. It is possible to use the EC keys that are used in bitcoin for PGP like encryption.
What about multiple addresses like PGP? Any way for someone to use the RIPEMD160 hash of their public key to encrypt the message (assuming the recipient has access to the private key?)
I do not thing it would be possible or secure to encrypt something to the RIPEMD160 hash since encryption requires knowing the public key and the hash does not reveal the public key.