Back when I was running 87 Octane and had a full tank of it and wanted to run my 93 Octane Tune I wanted to (Sipon) the gas out and use it in my wifes car. I asked AM if I could do this and they said No, you cant get the gas out this was. So Im assuming that the pen did'nt make it that far down the filler neck.
Since the spring is Metal and so it that little tip on the Pen mabey you can get a Mechanics Maganet and fish them back out??
If not try what was mentioned above. But I'd ask someone who know's how the filler neck is attached and if there's any way anything could get into the tank first.
A little TIP. When I add "Stabil" in the fall when I store my stang I have a piece of Rubber Garden Hose about 8" long, I attach it to the Stabil bottle neck with a Hose Clamp, it works slicker dan Poop, also works great on adding Fule Injector Cleaner
Good Luck and let us know the outcome.