I am considering mod support in my game as well. I came across LuaMachine which looks linteresting.
Essentially you export your C++ code to Lua and give them only whatever interface into your game that they need to implement the functionality you wish to be mod-able.
They then don’t need access to your engine code, they just need access to documentation about how your Lua API works.
I haven’t used this yet, I just saw it in the marketplace and thought it looked interesting.