Dynamically augment existing syntax at a position on screen

I've got a file with syntax highlighting.
What I'd like to be able to do, at any give line&column
character position on the screen which has a current
syntax highlighting scheme is to augment the highlighting
via a command/function at that one place, say, by
adding an underline at that point.
Is it possible (easy or hard) to do this?

Thanks
Richard

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