It’s a fair price to pay, but I think the suggested change is better because it makes you pay less for the same benefit. It still would be obvious that there’s a lock, but the code would IMO be simpler:

  thing->field.with([&](Field& field) {
    use(field);
  });

would become

  {
     auto field = Mutex.locker(thing.field); // or thing.field.lock();
     use(field);
  }

or

  use(Mutex.locker(thing.field));

Yes, that requires you to understand destructors exist, but if you’re programming C++, that’s a given.

I find that easier to understand (partly because the first doesn’t mention ‘lock’. I don’t think ‘with’ is the best name there)