Thank you McCreed for the suggestions.
And thank you for sharing the experimental technique with baking soda, but I might leave it out of the equation for now.... and you were right to suggest the plastic rods option as first choice!

Today I got some wood filler and did some for inlays tests of some hardwood I have here hanging around (not rosewood).
Well... The results are a bit mixed. Some dots look really good and believable, other look a bit weird and uneven. The white filler over a dark wood it's not very forgiving, for my current wood skills at least.
I think that I will abandon this idea and go ahead with the plastic rod. I did a test with some optical fibre I have hanging around and it's much easier & quicker & neater.

I have been scouting around the house to try to find a little white plastic rod somewhere, or grab it from an old toy or similar... The idea if buying plastic doesn't sit right with me. Will keep looking.