I reckon you'd get the cow hitch undone with no damage if you were patient. I did one once after 6 years of heavy strain in 16mm dacron double braid (premium quality).
I first soaked it in cold water
for a while, changing the water
, to extract the worst of the dried salt
(which softens it considerably) Then a few warm cycles, the last with a splash of dishwashing liquid (the old school
sort, NOT washing
Then it was a matter of 'breaking the back"*, alternating with wriggling a small fid through, under the 'arch', along the groove between the parallel standing parts
A fid with a slender point and a long slow taper is good. Any metal turner can whip you one up out of stainless. Tell them it's "OK to reposition the compound several times" if they say they don't have a taper turning attachment. This would not be kosher for most tapers a machine shop produces, but more than adequate for a fid. (Possibly this is more of a marlinespike - I can never remember the diff)
*Breaking the back refers to forcing the 'arch' away from the clew, while hanging onto the two elements of the standing part.
If you have a portable vice (or can take the sail to a fixed vice) this is a lot easier
Thumbs are what I used to break the back, I used to have a strong pair, but I didn't have much left in the way of fingerprints by the end of this particular job: now I'd be inclined to use a rubber mallet. Failing a vice, a sandbag on the foredeck is nearly as good. (well, fairly good)
NB: The vice needs smooth jaws, or thick adhesive
tape applied to soften the diamond knurling's potential to damage the fibres, maybe aluminium tape if you have it, or rubber rather than plastic which is obviously slippery
either way is a lot more convenient than smooth packers, because the rope
will keep needing to be regripped as it slips, but proper 'soft jaws' would be fine.
The same method (breaking the back) works for other knots, especially a bowline (or its structural equivalent, the sheet bend) which can -- if you have unlimited time -- virtually always be undone regardless of duration and/or strain
ON EDIT: I'd skimmed the last couple of posts - looks like the marlinespike has been thorougly canvassed. Never thought of using a billfish bill - talk about exotic.
I'm a softy though, mine would have to have died of natural causes, incl predation by any other predator than my fellow top predators.