I can't find any resources about BCP(Bridge Control Protocol) with Juniper on the web, I'm currently using SSG 140, anyone know if it supports BCP?
In JunOS (as ScreenOS is unlikely to get new features) BCP isn't present.
The official Juniper answer on how to do this would be to use MPLS and martini tunnels (CCC), or VPLS. This is a right pain to configure the first time, and more so cross-vendor (especially with Cisco as you have to figure out their licensing).
Honestly my preferred suggestion for someone not capable of running MPLS would be to convert the links to Ethernet and use VLANS (and perhaps QinQ).
Why do you want to tunnel ethernet over a PPP link anyway?
Why not just route? If you have overlapping address space or similar issues what's wrong with an IP-IP or GRE tunnel?
If you really do need BCP it appears some of the Mikrotik products have it, and they're cheap enough to buy for experiments.