There is a nifty tool called a Kwik Klip Basting Tool. When I first saw it I though it was stupid, after all I can close a safety pin just fine, thank you very much! But then I discovered, like you did, that you can't get your hand under there, and my fingers were getting very sore. Once I tried the tool, I was sold!
You hold the safety pin in your dominant hand, poke it through the fabric, and holding the Kwik Klip in the other hand, when the point of the safety pin comes back out the top of the fabric, you use the Kwik Klip to fasten the pin. Easy peasy! You can see it here:
http://www.andicraftsquilting.com/sh...sting-Tool.htm