After much dicking around and worrying about removing the entire intake etc., I made up a compact 15/16" crowfoot socket - basically a wrench cut at the jaws with a 3/8 drive welded on and shrunk on the grinder.
After confirming said modified socket would actually be able to grip the brass fitting and be able to move it at least 1 flat at a time, I armed myself with a couple of 3/8" extensions, ratchet and a long pipe, got the socket thing in place on the nut and managed to remove the stuck-and-leaky heater control valve.
Only minor lacerations to the back of my hand BTW, but blood thinners and coolant did make it seem worse than it really was.
Feeling pretty chipper that I got it off, I grabbed my spare from the parts car and in no time flat the new valve was on and working beautifully. Took it out for a test drive and the A/C came on when I asked it to and so did the heat - perfect!
Stopped by the shed to drop off the tools and opened the hood to have a last proud look at my work but … …drippy drip drip drip … the replacement valve leaks!
Is it possible to get a new one anywhere in this world?