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I also have all those paranoic thoughts about fast string processing:)
And so I've decided to do this (I didn't compare performance):

After opening Lua state I put table like this into registry:
    lua_newtable (L);
    lua_setfield (L, LUA_REGISTRYINDEX, "*prim*");

and in C function I look up for this table like this:

   lua_getfield (L, LUA_REGISTRYINDEX, "*prim*");
   lua_pushvalue (L, -5);
   lua_rawget (L, -2);
   if (!lua_isnumber (L, -1)) return failure ();

At least I don't need to think about strcmp () vs strliteralequal ()
which is declared like
#define strliteralequal(str,literal,n)                                  \
    ((((sizeof ("" literal)/sizeof (char)) - 1) == (n)) && !memcmp
(("" literal), (str), ((sizeof ("" literal)/sizeof (char)) - 1)))

It includes embedded zeroes during comparison.