I don't believe this is possible, although Juniper's documentation is very vague.
It is my understanding that in order to do FCoE, the EX4550 must be a stand-alone device, not part of a Virtual-Chassis.
There is vague reference in the EX4600 doco that says:
Note: Standalone switches support FCoE. Virtual Chassis (VC) and mixed-
mode Virtual Chassis Fabric (VCF) configurations do not support FCoE.
Pure QFX5100 switch VCFs (consisting of only QFX5100 switches) support FCoE.
This can be found at the following URL:
http://www.jnpr.net/techpubs/en_US/junos13.2/topics/concept/fibre-channel-fcoe-understanding.html
You could test on a dummy VLAN on your EX-VC by trying to commit the following and see what happens:
ethernet-switching-options {
secure-access-port {
interface xe-0/0/0.0 {
fcoe-trusted;
}
vlan my-fcoe-vlan {
examine-fip {
fc-map 0x0EFC03;
}
}
}
}