Hi
I have an idea...
The radiobuttons are placed on a panel, right?!?
So my thinking is, why not use the panel's 'canvas' to paint a green line under the selected radiobutton, I mean the radiobutton's 'ON/OFF' is a boolean, so one can run over them in the panel's 'OnPaint' and 'Greenlight' the chosen one.
With a bit of math to figure out the position of the line, it could work nicely =^
Just a thought

Regards Benny