[NTG-context] highlighting chars on a path

Hello, I am new to ConTeXt and I have looked on the internet for a way to do something special. Unfortunately I have not found any solution. I hope this is the right place for such a question. I apologise otherwise. I would like to highlight (e.g. put in bold, or change the color...) any character on a page which sits on a chosen path. I would like to be able to position for instance an invisible circle on a page and have all the characters "in contact" with this circle change color so that the circle become visible via these characters. Is it something I could do with ConTeXt? Thanks in advance. Aurelien