The Confederate Congress once passed a resolution declaring that all captured "negro or muttalo" Union soldiers should be tried and executed for "inciting servile insurrection". In response, the United States declared that regardless of any pretense of trial carried out by the Confederacy, *all* Union soldiers were under the protection of law and that Confederate POWs would be executed for any Union prisoner of war executed by the Confederacy. Likewise, Confederate prisoners would be placed at hard labor for any Union prisoner enslaved.
Ukraine should do the same. Treating prisoners of war in bad faith is something that any civilized nation *must* take a hard line on.
Edit: The Confederacy did not rescind the policy, but in practice mostly backed down and left the treatment of black POWs to individual commanders. While the traitors' behavior towards these soldiers did remain consistently inhumane and dishonorable, they never had the guts to follow through on mass executions.
This may well end up how things go, but i think your counter plan would be a tactical error, considering the very different political situation of Ukraine and Russia.
Considering most of the news we have seen, we can safely say that Russia,
unlike Confederacy, does not care much for the well being of its own soldiers. Its not even new, it goes back to WW2, when liberated POWs were often sent to gulags or penal battalions, and surrender was openly criminalized.
So for one the threat of execution would mostly serve to help Russia save on Chechen barrier troops if anything.
Secondly, many of Ukraine's allies are soft hearted humanitarians and would struggle to deal with that, especially after Russian propaganda catches up on exploiting this.
Neither of these problems can be easily removed.
I think the more realistic countermove would be to capitalize on the western ally control of certain international institutions and using that to deliver a blow where it hurts, as in Russian mythology about their WW2 history. Namely,
as a mere warning, equate the status of Russian armed forces members in 2022 in international law with that of Waffen SS after WW2. So, goodbye to any vacations in western aligned countries ever, if they want to avoid the humiliation of slow investigations and interrogations about whether they were aiding and abetting war crimes in Ukraine. Not avoidable for the POWs though. Above all, Russian media and politicians would be stuck with the ungrateful task of having to explain to their sheep why most of the world is treating their heroes like the Nazis themselves.
As a side benefit it might get some extra intel through offering crown witness like status to soldiers who provide intel on said war crimes and other Russian state secrets for asylum in the west.
That would hit Russia where it hurts, its current main narrative of national pride, their prized and carefully managed "muh war against fascism" story.
And that would be just a warning...
So what's the main threat?
A declaration of total propaganda offensive against Russian sensibilities, namely western countries and institutions going full ham on equating the Soviet Union with the Third Reich, and communism with nazism, like by some accounts should have been done all along.
That would be harder to get westerners themselves to get onboard with than the warning, but still easier than to get them to stomach retaliatory executions of war prisoners.