How come I miss all the fun ones when they are first posted??
I would have to say that the odds of getting it all the way through the line to the pump are astronomicly low ! The way to get it back in reach (though not out, yet) is to apply lots o pressure/ volume to the pipe ie, one of the vac/ blowers the 'pros' use to set liners and blow lines. This should get it back up to the threads (but on the wrong side of them). Once you can reach it, loosen the wingnut (ie, decompress the plug) - do not remove the wingnut, it's your best grip on the plug. If you want to keep blowing on the line, you can, but you should be able to pull it out with vice grips. (Although some of the other advice already given might make an easier job of it)
The biggest problem I can see is aplying the pressure only to the skimmer line, though CarlD's answer covers hacking into the pipes (the connectors he was talking about are called Fernco's or 'no hub couplings' and should be available at any decently stocked hardware store). If you are going to cut the pipes at the filter system, install a valve on each line before reconnecting them. As it will be Sat before you 'do battle' with it again, you've got plenty of time to plan your strategy. If I can be of further assistence, or if what I've said leaves more questions than it answers, I'll check this post daily. Good Luck!!