[Ducay]

Based on my assumption of the perspective in that picture, the brass pipe is for the overflow? If you haven't already, most tubs (I've seen), the main drain's flange (the thing with the grate) is threaded and that is what holds the main drain to the tub. Usually you can just undo that, and undo the overflow (again, similar connection, you should be able to undo the tub completely from within the tub.) and then lift it out.

Now if that brass pipe is the main drain pipe - unless you can easily get at the coupling nut, I'd use a hacksaw or reciprocating saw on the brass pipe. (assuming that it is upstream of the coupling) and then undo the coupling when you have nice clean access to it. I'd rather be safe and F-up the brass pipe that goes into the garbage rather than the coupling you may need to ultimately remove.