that was kind of a stupid thing for me to say, since your problem involves your compressor. my bad.
i was just thinking off the top of my hairless head.
next best thing would be to take a hacksaw blade and grind down the top side of the blade so it would fit inside the fitting, then use that to slice a couple of slots into the broken part of the brass. the slots do not really need to be 180 apart, just enough so when you can get the first chunk of the fitting out, there is enough room for the rest of the fitting to collapse and be small enough to exit the tank threads.
you could also use a jig saw with a fine tooth metal blade to make the cuts. use the low speed if it has that feature, or if the variable speed kind, be "gentle" on the trigger.
use some grease while cutting regardless of the method, to keep the most particles of the cut you can out of the tank.
whatever method you use, please let us know how you make out.