Well, it's easy to say "always go prepared" but sometimes you just can't. We have a friend who recently had a Rottie litter and whenever we're together, she expects me to shoot the pups. And the conditions are almost always as you described. So, I take them outside and let them play while I lay on the grass and eventually they come and play with me as well and we get some really cute stuff. The best advice I can offer is always have someone who can "bait" the puppies and whom the puppies will react to. Use anything you can to get their attention -- toys, rude mouth noises, whatever. And find someplace that is acceptable for being in your images. Oh, and clean the puppies up. I've washed my fair share of puppies that weren't mine (and all the ones that were) and that makes a huge difference especially with white dogs.

Good luck.