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, but it is generally a bad idea.
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 don't believe so. RIPEMD160 is a hash, which means that information is lost as the input is reduced to a digest. I don't think there's enough information left in the digest to be useful for encrypting a message that can be decrypted with the original private key.