Thanks to all for suggestions.
We finally got this apart. Thanks for all of the advice. Here's what we did:
First I saturated the area with calcium hydroxide on Saturday - and kept it wet with a cloth. Calcium hydroxide has a pH of 12 (on a log scale of 1-13, with 7 being neutral). Calcium hydroxide is pickling lime, or you can find it at pet stores where it's called "Kalkwasser" - it's used to increase the pH and calcium in a reef tank. It doesn't take much, it saturates at 1 tablespoon per gallon.
Then, yesterday, I heated up the tubing with a torch, and sprayed the casting with CRC Freeze Off. A bunch of wacks with a screwdriver and hammer, and it came apart.
San Antonio/Port Aransas, TX