I expect someone has done this before, but maybe not documented it.
I'd been wanting to try it for ages, I'm doing a longbow and there are a fair few nots to plug.
An ordinary plug looks a bit obvious with straight grain going across it, so I used a small branch of Yew to make a plug from the heart wood with it's natural circular grain. The plug looks just like the original not, but it is clean and solid, where the original had a black powdery layer all round it, so it wasn't solid with the parent wood.
I cleaned up the area around it to show it off... the bow is still a long way from finished.
The pic shows the plug and the small branch it was filed from.
Sorry if this is already well known trick

The knot goes in towards the sapwood an makes it bulge, but had almost grown over leaving just a tiny hole showing. I cleaned out the hole to sound wood and loaded it with yew dust/epoxy mix, so that when I pushed in the plug it filled any voids and oozed out of the small hole in the sapwood.
Del