Untested LuaMeta crap

lua_newtable(L);
    lua_pushcstring("SWEP");
        lua_newtable(L);
            lua_settable(L, -3);
    lua_newtable(L);
        // stack is now env, meta
        lua_pushcstring(L, "__index");
            lua_getglobal(L, "_G");
                lua_settable(L, -3);
        lua_pushcstring(L, "__newindex");
            lua_getglobal(L, "_G");
                lua_settable(L, -3);
        lua_setmetatable(L -1); // meta is set, env now on top
    // stack is env
    if(lua_loadfile(L, "file.lua"))
    {
        std::cerr << errormsg << "\n";
        lua_pop(L, 2); // pop err str AND env tbl
        abort(); // or continue or something
    }

        // set the env
        lua_pushvalue(L, -2); // the copy the env table to the top
            lua_setfenv(L, -2); // pops -1 and sets as env for -2

        // now the env is set up, invoke it
        if(lua_pcall(L, 0, 0))
        {
            std::cerr << errormsg << "\n";
            lua_pop(L, 2);
            abort(); // or continue or something
        }
        lua_pop(L, 1); // was called above, don't need the loaded func anymore

    // the table is now on the top of the stack (-1)
    // get the SWEP table using lua_gettable
    // ....

    // we've done all now, don't need the env table anymore, let's return
    lua_pop(L, 1);
// stack is now clean