Sounds great!

Not sure about the method layout. What you have sounds good for literalDataPacket.
But Message and CleartextMessage is on a different abstraction layer.
So setText for a message is currently openpgp.message.fromText, which means
you create a new literaldata packet, put it in a packetlist and this forms the new message.
setBytes, and getBytes feels also to low-level for me for Message and CleartextMessage
How about openpgp.message.fromBinary similar to fromText and it returns a new message
object? getBytes could stay getLiteralData.