Sorry, I don't have any advice on that seeing as though I haven't ever done bondage. I'm only 18... 
I just seem to be thinking in terms of professional bondage that you see in porn. Don't think it'd be easily replicated at all... I say change her position. If you're going to do bondage, do it right and don't untie her. Just to keep all that excitement with restraint locked in. 
Right now I'm thinking have her hands tied together behind her back and her thigh and leg tied so that her legs are folded (I know, bad description...) so that she basically can't walk until untied. I hope that makes sense. Just another idea that I may be wrong about.
OR... if she's flexible enough... have her hands tied together above her head and her feet tied to her hands. Basically, her groin will be right up in the air and that would call for a little more brutal penetration (again, bad description... I think you know what I mean with 'brutal'). Depends if she feels comfortable in that position or not...
Actually, I suppose I do have some sort of advice, then. Shows how long I spent on this message as two has popped up whilst I was writing it. WELL! Off to the Royal Mail whatever building to pick up my toys hopefully. Been waiting from 9:00 and near 1:00 I got impatient and decided to get food. Come back. Damn... two damn minutes late for that darn delivery. My rant over. I'll rage quietly. 
Also, take heed of what Hella said. Just keep, in some way, your girlfriend restrained. I think up against the wall can be classed as restrained as she's unable to get out of the hold.