Hi Johan,
you are right in your finding regards EX3400. It does not support ERPS which brings you down to classic xSTP.
An option for lowering the impact for a break in the ring could be doing two links on each hop and bundle them together in an aggregated ethernet (ae) interface which keeps the logical link up in case of optics failure or repatch error in a patchpanel. Not pretty but better than nothing.
Alternatively you could look into some (passive) DWDM multiplexers with OADM's beside each EX3400 VC making it possible for you to connect all EX3400 VC's directly to the EX4600 VC via ae's. This would require single mode cabling all over and some extra cost for optics, multiplexers and OADM's...