We spoke to our Juniper SE team and they were surprised to hear that it's unsupported on the EX4600 platform. Interestingly, I also tested this on a QFX5100 with the same results. I guess that makes sense because the QFX5100 and EX4600 are pretty much the same device.
On another note, we ended up accomplishing our goals with some static/host routes. Not exactly elegant, but it works.