This is one of those jobs that's been on my to-do list for ages. Not Plastidip, but getting them sprayed properly in satin or gloss black.
I think once they've got to a certain point, ie that lovely dull grey colour, you'd forever be chasing it with any kind of trim restoration product, and Plastidip might not be that durable.
There are a few old threads knocking about with photos of sprayed grilles, and to me they look spot on. A bit like those on modern RS Audis. But like Luke says, it might be fruitless trying to find those threads!