I.e. the place where the class member name normally is shown (cells with gray background)
Go to Edit | Preferences > Results and choose custom results style sheet. Then add this:
My be I was unclear I want to do a little more than change the background
I want to add a checkbox that is connected to my c# code
Your only option, then, might be to use the LINQPad HTML controls to write your own table. There's a sample in the built-in Tutorial and Reference ("see LINQPad Controls"). Formatting the table might be too much work, though, depending on what you want to accomplish.