If you're looking to keep the collar and threads, I'd say try drilling / cutting out the part that you don't want. It'll take a lot of time with a dremel but you should be able to cut out enough of the parts you don't want that you can get the stem out.
Just make sure to keep venting that bad boy until you get a hole in it...
Building a Bad News Brewery - eHERMS
2014 - 70gal
10gal American Pale Ale
5gal Wee Heavy
10gal Irish Red
Keg 1: Apfelwein
Keg 2: Irish Red
Keg 3: Hefeweizen
Fermenting: 1gal Joe's Ancient Orange Mead & 1gal Traditional Mead