I am definitely open to PRs, the main function of frag lighting is outside the addon, the addon is just setting a flag for the hacked vertex shader in the lighting case. Shadow map would also probably be a little outside the addon, and typically requires multiple renders, 1 per light to create shadow map. Something C3 is not really setup for.
Instead would look at baking shadow maps and applying to geometry and use an old technique like this to roughly apply it to moving actors: https://www.reddit.com/r/Unity3D/s/jEOZBCfUSC
Have not tried it myself, but seems plausible.