According to the mod_lua documentation, there should be a function called r:parsebody, which parses the POST data and converts it into a Lua table, but I have yet to actually find this function declared inside the mod_lua code.

With 2.4, adding this functionality should be pretty straightforward: 
/* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ */
/* r:parsebody() returning a lua table */
static int req_parsebody(lua_State *L)
{
    apr_array_header_t          *fields;
    int                         i;
    apr_table_entry_t           *e = 0;
    request_rec *r = ap_lua_check_request_rec(L, 1);
    lua_newtable(L);
    lua_newtable(L);            /* [table, table] */
    ap_parse_form_data(r, NULL, &fields, -1, MAX_STRING_LEN); /* Maybe increase this value? */
    e = (apr_table_entry_t *) fields->elts;
    for(i = 0; i < fields->nelts; i++) {
        req_aprtable2luatable_cb(L, e[i].key, e[i].val);
    }
    return 2;                   /* [table<string, string>, table<string, array<string>>] */
}
/* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ */

(The function would also have to be declared in ap_lua_load_request_lmodule)

I hope this little snippet will at least give some food for thought, and I hope that the issue can be resolved at some point in time, as we clearly need more Lua in the world.