Is there a word, phrase or allusion which represents the opposite of a Pyrrhic Victory: a tactical defeat which led to a strategic victory, either accidental or intended? After all, there must be one for almost every Pyrrhic victory.
If the victory was so costly it led to defeat, then its opposite would be a loss that was so advantageous it led to victory:
2(In chess) an opening move in which a player makes a sacrifice, typically of a pawn, for the sake of a compensating advantage:
Pyrrhic Defeat Theory suggest increasing power by increasing the cost of a battle:
the idea that those with the power to change a system, benefit from the way it currently works.
In criminology, pyrrhic defeat theory is a way of looking at criminal justice policy. It suggests that the criminal justice system’s intentions are the very opposite of common expectations; it functions the way it does in order to create a specific image of crime: one in which it is actually a threat from the poor. However, to justify the truth of the idea there must be some substance to back it up. The system needs to fight crime, to some extent at least, but to an amount only to control it and ensure it stays in a prominent position in the public eye, not enough to eliminate it.
en.wikipedia.org emphasis mine
Attrition warfare employs the notion of pyrrhic defeat.
History records a number of heroic defenses that resulted in short-term defeat or even disaster, but that either wore down or delayed the enemy—and inspired allies fighting in the same cause—and thus contributed to subsequent victory in the larger war. Of these some have become bywords for what might be called "Pyrrhic defeats": "a Thermopylae," "an Alamo," "a Bataan."
However, some such defeats don't yield the later payoff—for example, (from a Southern Confederate perspective) "a Vicksburg," or (from a French perspective) "a Dien Bien Phu." A contemporaneous commentary following the battle of Verdun—"The End of an Illusion," from the New York Tribune, reprinted in The Bulletin (August 1916) is instructive in this regard:
This is what Lloyd George means, this is what Allied diplomacy and statesmanship mean. This is the voice of England, of Russia, and, above all, the voice of heroic France, which has borne so far a wholly disproportionate burden of the losses and the suffering, but has borne it with a heroism that will remain forever memorable and with an endurance unshaken after nearly two years. Verdun will yet rank as a second Thermopylae in the history of our civilization, a Thermopylae which surpasses the ancient in the single fact that it was a victory, and not a defeat, since the Barbarians did not pass.
Today it's difficult to think of the carnage of World War I as anything but a pointless slaughter of soldiers and civilians from every nation concerned. I doubt that anyone looks at Verdun and says "Pyrrhus would have loved the effect that battle ultimately had on the outcome of the war."
You might say it was a suicide mission, or that that group made a sacrifice for a more critically important success; they were sacrificial lambs. In the muzzle-loading rifle days you might call your doomed first wave assaulters the forlorn hope. A more aggressive suicide mission might be described as a kamikaze attack.
I think what you have in mind is not the "Pyrrhic Defeat". Pyrrhus scored a victory against the other side by sacrificing too much, almost losing. What the losing side is experiencing is the feeling "we lost but we almost took them down with us". The example you give with the separatist party seems more like an unwitting victory to me (obviously referring to the influence gained and not the referendum).
You could, perhaps, borrow a term from the world of commerce and say "The battle of xxx was a loss leader", or "xxx was a loss-leading battle".
From the OED [paywalled]:
loss leader n. Commerce
an article put on sale at a non-profit-making price in order to attract potential buyers of other articles;
(Interestingly, to me at least, I assumed this was a fairly modern phrase, but the OED's earliest citation is from 1922).
This would primarily cover the intentional aspect of the original question (a well-managed company shouldn't accidentally offer goods at below the break-even price), but I can envisage the phrase also being used for the accidental case ("We didn't know it at the time, but the "no" vote in the referendum turned out to be a loss leader.").