Mwaaa Ha Ha ha Ha aaa HHHaaa Ha ha!
The Gods of Plumbing smile upon me this day!
I started experimentally stabbing into the foam insulation. It really wasn't as far to go to hit the interior tank as I thought it would be.
The pipes are sort of off-center in the hole of the sheet metal cover. So, even though there was little room on one side of the pipe, it made more room on the other. I took advantage of that with a skinny chisel, and cut some foam out.
As Xringer said, sure enough, I could see some exterior threads on the other end. I also did NOT see any sign of solder/welder/weird gaskets/etc.
Time for the pipe wrench!
(Deep Breath!)
(And turn it....)
Yea! It unscrews!
On the port that the brass nipple went in to, it was just 3/4 female thread, then it shrank down to a smaller size (1/2"?) for what looks to be the actual heat exchanger tube.
On the second pipe, it was much tighter and more difficult to remove. Had I not already removed the first one, I wouldn't have tried any harder on the second one. I would have assumed I was screwing it up!
All I really needed was a longer handle for my pipe wrench!
Here's the two hunks of what I pulled out!
It all seems so simple now, but not knowing what was at the other end, realizing what the material was and NOT WANTING TO SCREW UP MY tank made this all very mysterious to start with.
I also noticed, after I polished up the cut pipe, that there were some indented lines running parallel to the length of the pipe.
I couldn't figure out what they were. Part of the manufacturing process?!?!? Oh well. After I used the pipe wrench to get the nipples OFF, I realized I just made MORE of the EXACT SAME MARKS! Those were marks from a pipe wrench when they put those nipples in!
Had I more experience with plumbing, I might have known that and realized right from the start that these were just threaded-in pipes!