Can I prove it? Yes.
So prove it then. I'll wait.
Show us all beyond a reasonable doubt that you can effectively point out in any given situation who is actively using this mod against you.
What do healers do?
If you think healers actually blend in, then you're sorely mistaken. Blizzard has gone out of their way to make healers as obvious as possible. Nearly every healing spell has bright glowing colors every time you cast a heal, which is clearly different than the color of a damaging spell.
Disc priests create large, easily seen bubbles around their allies, and have a big glowing ball of energy pulse at their allies repeatedly as they heal them.
Holy paladins new multi-target healing spell similar to wild growth shoots blinding rays of light emanating from the caster.
Resto druids are the easiest to spot. Pre-cata all you had to do was roleplay a lumberjack. These days anyone wearing druid armor in caster form is a target, because the other two specs have primary shapeshift forms.
Any shaman wearing a shield that's glowing green, yellow, or white is obviously resto. Especially if they're shooting bright yellow jesus beams at people which track directly back to the caster.
Anyone wearing priest armor that's not a dark shade of grey obviously needs to be killed.
I could go on and list so many other abilities that jump up in your face and shout "I'M A HEALER, HERE I AM", but you should get the idea by now. To think that you're hiding anywhere as a healer is foolishness.
What is this add-on doing then?
Stopping healers from blending in.
See above, healers don't blend in, at all. The only people you think you're hiding from, are the incompetent morons. People who wouldn't know how to kill you regardless.
I'm not saying this is happening to me, I've already stated I don't PvP on this toon.
Then what is your problem? If it's not happening to you, and you don't PVP on that healer, then let other people speak for themselves. Stop trying to be a white knight and join in on a crusade that doesn't affect you either way.
Perhaps you should play a healer sometime to actually get a grasp on why most healers commenting on this topic are telling you that you are wrong. Play PvP as a healer without staying with a big group or hiding in those bushes that are out of the way.
More baseless assumptions. You have no idea what the poster you quoted does when they PVP. As for myself, I frequently play a healer in PVP. That's about the only time that I'd heal, as a PVE healing I find boring.
Even on hybrid characters that are primarily DPS specs, I'll still immediately check the health of nearby allies that are already engaged in the fight I'm about to join. If any are critically low I'll cast off a couple quick heals to keep them up for a bit longer before joining in myself.