Дмитрий Сергеев

оказалось всё просто

location @lua_call {
rewrite ^ /default.jpg break;
rewrite_by_lua_file /var/www/site/www/images/hello.lua;
# Отключим кэширование кода для разработки
lua_code_cache off;
}

читаем доку https://github.com/openresty/lua-nginx-module#rewr...

This will be the case if rewrite ^ /bar last is used as this will similarly initiate an internal redirection. If the break modifier is used instead, there will be no internal redirection and the rewrite_by_lua code will be executed.