To the OP, this is not intended to be overly critical, but triggers are serious business, so serious discussion is warranted.
Other posts here have addressed the original question about tumbling trigger parts with walnut media, but I want to address the broader issue about looking for shortcuts to improving triggers. Trigger function is critical for safety and reliability, and triggers have sensitive tolerances. There are no shortcuts for getting a good trigger. This is an area where gunsmiths really earn their money.
The basic idea behind trigger job "shortcuts" is to improve the trigger without having a detailed understanding of how it works. This plan is both unsafe and unlikely to work. For all types of trigger work, it is necessary to polish or remove metal in certain areas while maintaining safe tolerances in other areas. Striking this balance requires detailed knowledge about triggers in general and about the specific trigger you are working on.
Now that the question of "shortcuts" has been addressed, I want to discuss reasonable expectation for home gunsmiths. There is a big difference between double action triggers and single action triggers. With careful study, detailed handwork, and some simple stones, double action triggers can often be improved by home gunsmiths. Improving double action triggers generally involves smoothing a variety of surfaces, and the tolerances are not too precise. However, it is still important to be aware of surfaces that should not be touched. Note that this process will involve study and real work, not shortcuts, but it can be achieved. If this does not sound fun, then talk to your gunsmith, he has already put the work and study.
Single action triggers are a different animal. Only a few small surfaces are involved, and the differences between a poor trigger, a good trigger, and an unsafe trigger are just a few thousandths of an inch and a fraction of a degree of angle. Working on single action triggers is beyond the capability of most home gunsmiths.