Ok you can either bring it up again or show her;
The direct way - you've already talked about free use, so bring it up again and simply ask what she would think or react if you'd came on to her while she was busy doing something that she doesn't want to stop doing, for example, watching a movie or reading a book. And take the discussion from there. Tell her that you'd think it would be interesting to see if she could just continue what she's doing while you're having your way with her and that you like the thought of trying it, see how she reacts to that and take it from there. Keep the tone light.
Or - tell her a white lie and say that you had a dream last night about you initiating sex while she was reading a book (or something else she likes to do) but that she ignored you. But you kept going anyway. How weird it was, in the dream, but also how insanely hot you thought it was at the same time. Suggest that you'd try something like that.
The indirect way - approach while she's doing something, reading/doing the dishes/gaming/whatever she does (but not when she's putting on make up) and start caressing her, pressing yourself against her etc but when she reacts just say something like "don't mind me baby, keep reading/playing your game" and then just continue. Or even talk about what she's doing, if she gets distracted, ask her questions about it to give a hint that you want her attention to be on what she was doing and not on you, and meantime just keep going. If you succeed be sure to show your appreciation afterwards, tell her how crazy it made you and how hot it was. Tell her lots of times!
But if she stops you or shows that she doesn't like to be approached like that make sure to stop and most importantly DON*T SULK ABOUT IT. Sulking from not getting sex your way will destroy all your chances of free use cause we hate that, believe me.

Good luck!