Great post Nick! I have been trying to figure out how to get some relief for my stiff knobs too - on a DIRZone twin manifold (I think these are similar/identical to the Agir ones made by Scubatec in Italy - not sure though).
On Friday afternoon I stopped by my dive school's equipment maintenance room and (with a bit of guidance) disassembled the valve and added a little extra O2-friendly grease to the threads (there was a bit on there already, which was good to see). I had the same results as NickB - they were nice and easy to turn with no gas in the tanks, but just as stiff as before once pressurised.
Was going to post and ask for suggestions/links, but you beat me to it mate. I'll be reading any other replies with interest - anyone else done this, got any other solutions, reckon it's not a good idea?