Ok, first round of experiments - moderate success, not overwhelming though!
I soaked into in hot coffee/tea with the grounds and leaves and left well along for a few hours. then i took them out and let them dry. Finally i wiped them over.
Start and finish pics below, plus a couple of inprogress shots

So there is a little difference but its certainly not huge. I think there are two main issues, the shiny plastic surface and the time i left it all for.
So i took them out and fine sanded the bits you would expect to see more wear and its all gone back into the grounds.
The problem seems to be that it wil make them look dirty but it wears off a little quick - hopefully the fine sanding will help it stick better.
So far it looks like this method could work but i dont think it will be as controlled as the wirewool and tinted wax method