One strategy would be – “use BI!” Or, “use BI and Cycles.”
BI is a ray-tracer algorithm that effectively goes from a light-source to see what the light from that source “hits.” If it is a layer-specific light it “sees” only objects in the chosen layer(s). I like to say that it’s a good “spotlight.” (Or, “spot shadow.” Yes, you can have a spotlight that casts shadows but not light.)
Cycles is a path-tracer which effectively searches out in all directions from an object to discover what things illuminate it, and for pragmatic reasons it must consider everything. It uses the GPU to perform massively-parallel computations. It’s a good “soft box.” The lighting is accurate and beautifully even … which can get a little boring.
But you could use the two together, and composite the result – rig spotlights in BI to shine light on exact things and then comp the two renders together. (BI wouldn’t be doing the whole thing – just the “spice.”) This gives you far more precise control of the illumination by leveraging the special capabilities and viewpoints of both algorithms at once. Real-world photographers often use both soft-boxes and strobes for the same reasons.
And incidentally – sometimes what you really want from BI is those “spot shadows” that I mentioned. (Shadow information can also be extracted separately from a render that uses regular lights.) A slight darkening of certain areas, consistent with the presence of a clearly-directional light source, can be exactly what you’re looking for to add a sense of drama and depth to an otherwise-Cycles render, without adding any “light” at all. (You can fine-tune the effect in “comp,” using the shadow data for a variety of creative purposes.)