Here are a couple of examples of how the added grain looks like (they correspond to two possible choices of the grain size), compared to the original image.
The difference with respect to a classic grain overlay might not be huge, but is visible. As an example, you can find below a comparison with the result of Darktable's grain filter at 6400 ISO and 50% strength (mouseover to see the result of my method):
All that might be just over-complicated, but it was an intresting intellectual exercise which brings (at least to my taste) some visual benefits. Probably it will land into PhotoFlow sooner or later, also depending upon the feedback I get from possibly interested users...
UPDATE: G'MIC also provides a nice grain simulation filter, and it was interesting to compare it with my own recipy.
Surprisingly, my "large grain setting" shown above matches very closely G'MIC's "TMAX 3200" preset at 80% of opacity, 100% scale and "grain merge" blending. The comparison is show below (left: G'MIC, right: my code):
Below you can see the test image rendered by G'MIC using the same settings (mouse over to see my own result). As one can see, while the grain rendering in the mid tones is very similar, the two methods give quite different results in dark and light areas, as well as regions of high contrast.