UPDATE:
Reply directly from Juniper ("Sr Partner SE" person) for reference:
"We have the number of switches supporting DCBX with ECN, but there’s no explicit support for RoCE yet.
DCBX is just one of prerequisites.
As I have mentioned, the first our platform with RoCE support will be QFX 5100. I would consider seriously QFX 5100 – it has above mentioned features and there is a chance for RoCE support yet in this year. I see RoCEv2 support for QFX5100 scheduled for 17.4."
So seems no explicit support for RoCE on EX4600 ...