I'd think you could add a sub-panel, fed from a 30 or 40A breaker in the transfer switch, and add the additional lighting circuits to that panel.
If you use cfl or led bulbs, lighting loads are very low these days.
This is exactly my set up. I run a 16-circuit sub panel off a double 40-amp breaker in the transfer panel. They are all lighting/standard outlet circuits, so the over-all load is low.
Just ensure your grounds and neutrals are separated and there is no ground to neutral bonding jumper in the sun-panel.