Hi Folks,
I did the same with a MX box inbetween once [as L2], it uses "instance-type virtual-switch"; not sure if EX supports the same,
set routing-instances sw1 instance-type virtual-switch
set routing-instances sw1 interface ge-1/0/2.0
set routing-instances sw1 interface ge-1/0/4.0
set routing-instances sw1 bridge-domains vlan100 vlan-id 100
set routing-instances sw2 instance-type virtual-switch
set routing-instances sw2 interface ge-1/0/3.0
set routing-instances sw2 interface ge-1/1/4.0
set routing-instances sw2 bridge-domains vlan100 vlan-id 100
set interfaces ge-1/0/2 unit 0 family bridge interface-mode access << PC A
set interfaces ge-1/0/2 unit 0 family bridge vlan-id 100
set interfaces ge-1/0/3 unit 0 family bridge interface-mode access << PC B
set interfaces ge-1/0/3 unit 0 family bridge vlan-id 100
set interfaces ge-1/0/4 unit 0 family bridge interface-mode trunk << TO SRX
set interfaces ge-1/0/4 unit 0 family bridge vlan-id-list 100
set interfaces ge-1/1/4 unit 0 family bridge interface-mode trunk << TO SRX
set interfaces ge-1/1/4 unit 0 family bridge vlan-id-list 100