Yeah, it sounds like LRs will be what you need. You just need to find out what JUNOS release will support them on the security platforms. They should be renamed to Logical Systems by the time it is supported on SRX.
No transparent mode right now. Also, L2 switching and L2 control plane is not supported right now, as it is on the MX, J, and EX-series. Beyond Transparent Mode, I expect the high GigE port density of the SRX may drive requests for logical numbered VLAN interfaces and bridge groups so that multiple GigE ports can be unnumbered and be part of the same bridge domain.
Although most of my responses today have been "roadmap", the roadmap is very strong and adds new features with a schedule that permits the aggressive testing that people come to expect from JUNOS. More important, security customers now get 4 releases per year. It's predictable and makes waiting easier when you can get new features every single quarter.