interesting - I don't think you're imposing anything. On the contrary, I really like the idea. Mind you - as you said - only a couple of shots. They won't take that much time and could produce something unique, assuming you don't completly mess up the shots. Even if, you're not ruining the wedding in any way. The main photographer will take all important the pictures anyway. You're just a guest who happens to have a really big camera
Why don't you just ask them? Show them the camera, tell them what you'd like to do and if they're up for it? Ask the pro so you can somehow shoot together when they're doing the group shot, or it's just the two of them posing.
Seriously, I say go ahead and ask them. Maybe they like the idea? I surely would. I think it's a special day, so why not have an equally special picture to remember it by? Make them an album with the shots, do it for free, or try and get some film and paper money and leave it at that. I can't imagine that you'll make yourself unpopular with that.
If you're unobstrusive and quick - like you should be at a wedding anyway - why not?