Have you considered doing pre-pass lighting? It's basically the same as what you conclude, but for all objects in the scene. It uses less render targets (usually just one if you have hardware support for reading depth buffer) that classic deferred rendering, and also enables more variety in the materials (which is sorta that problem you're having).
Probably drawing the emissive mesh a second time is a nice thing (if there aren't too many of them), as it gives you the most flexibiltity. Imagine extruding the triangles and using some fancy-pants shader, you could probably have a volumetric glow rather than a flat emissive.
Thanks guys, you remind me of why this forum is awesome.
@jewzorek You're absolutely right, in the end this will be run on a mobile device, so space is critical. I just wanted to inspect so I can see where stuff is going.
Exe size sometimes does sometimes matter on desktop though, sometimes you want smaller instructions, as it will run faster than less arithmetic due to the instruction cache not being trashed. Loosely related to global exe size (only matters in hot loops though).