FYI, it is supported on the High-end SRX. Look for an application note named "SRX series services gateways cluster deployment across layer 2 networks" (google will find it - its somewhere on the juniper website).
We did such a setup with 3400s a while back and it takes quite a bit of work to get this up and running:
- if your switches perform IGMP snooping, try disabling it
- if the switches are cisco, you need to disable the ip-header verification. They will verify the IP header even for switched traffic, and the packets sent by the SRXs aren't valid IP so they will get dropped without any logging