Ok, so I add this to my device_luaprinter_interface:

It's a pointer to lua_engine in m_luaengine,
and also a pointer to a member function of lua_engine.

So I can call the function by

((*m_luaengine).*lualoadstring)(astr);
where astr is the string to execute.

Code

static void (lua_engine::* lualoadstring)(const char * astr);
static lua_engine *m_luaengine;

void callloadstring(const char *astr) { 
     printf("Trying callloadstring %s\n",astr); 
     ((*m_luaengine).*lualoadstring)(astr); 
}

and then in luaengine initialize them:

Code
device_luaprinter_interface::lualoadstring = &lua_engine::load_string;
device_luaprinter_interface::m_luaengine = this;

so now I can do this:

Code
[MAME]> l = manager:machine().lp[1]
[MAME]> l:callloadstring(" l:callloadstring('print(\"whoa!\")') ")
Trying callloadstring  l:callloadstring('print("whoa!")') 
Trying callloadstring print("whoa!")
whoa!