Anyway, I was able to get this to work at the Moonshade wardstone just next to the Porticulum. It had been taken over by a Life invasion, so we took it back. I targeted the 'stone, walked forward till the Bless icon turned red (because the stone was behind me). I stepped back until just as it turned white again, so I was pretty much in the dead center of the wardstone. Hit the power and bam.
I don't know if it was the ridiculous position or the fact we had just kicked an invasion force off it. Maybe because it respawned after the Life thing was destroyed (similar family of glitch to mining?) If this works for anyone else, post it, so the devs have an idea where to look.