Note that Java client is fast, well-supported, very mature and is built in a way that lets us reuse serialization code. Also, on the JVM hand-crafting serialization code is unlikely to bring as much benefit as it is with C: one more argument to reuse Java client's code.
amq-protocol seems to be using BSD/MIT-like license and Java client is dual licensed as APL2 and GPL2.
We ended up recommending everybody to use March Hare on JRuby because prior to their "9000" release, the I/O subsystem has enough subtle incompatibilities with MRI and bugs to be a real pain to maintain. So, this is no longer relevant since March Hare doesn't use this library.