In my original post above, I included a photo showing a 1/4 turn valve that used compression connections on both ends:
That particular valve leaked once the hot water tank achieves any pressure, either from city water connection, or heating the tank. I believe the cause is due to a groove in the ball portion of the valve that would not normally be exposed when the valve is installed normally. I am using this valve in reverse - using the smaller 3/8" as the input, rather than the 1/2" side.
I replaced it with a valve that is a sweat fitting on the normal supply side.
I still connected it to the 3/8" side, but the ball appears to be metal, does not have that groove, and does not seem to leak... at least, not in my limited test. Just thought I would mention the change in case anyone else plans to do this mod and was using my post as a "parts list".