The reason Colt will not take in any work on older guns is because if during any work something turns up that's defective or unsafe, Colt no longer has any parts to repair it.
That would leave them subject to the legal problems of returning an unsafe gun and following lawsuits.
Even if the gun was in for just refinishing or engraving, Colt would still be liable for a defective gun.
Example.....Colt takes in an original Python for a refinish and after disassembly finds the hammer or trigger has been altered and is no longer safe. Colt has no new replacement parts and is now responsible for the gun.
Another....Colt disassembles the gun and for some reason something breaks. Colt can't repair it and the customer holds them responsible.
I believe that a properly worded disclaimer signed by the consumer and sent in with the gun would fix that. I did some preliminary research and cannot find a single reported case at the State or Federal level where a gunsmith or mfg was ever found liable for declining to work on an unsafe gun and returning it to the owner. I would be interested in the source of your statement
Hammers or triggers that have been altered can simply be retained by the mfg and not returned. The idiots at Browning did this to me. I sent in a gun for an evaluation of ringed barrels. Browning declined to do warranty work on the barrels but before sending the gun back to me, swapped out the adjustable triple trigger that came with the gun and installed an unadjustable trigger. It kept the original parts and returned the gun to me with the new trigger. Not only were they wrong because the gun came with the triple trigger in the first place, but they installed the new one incorrectly because the top barrel would no longer fire. I had to buy a new triple trigger and pay to have it installed. I also had to pay a local german gunsmith to fix the failure to fire problem. By the way, browning claimed the ringed barrels were unsafe but still returned them to me. The german gunsmith looked at the rings and was unsure if these made the gun safe or not but still returned the whole gun to me. I took the barrels to a master barrel maker in italy who took the rings out and the gun was as good as new.
People mistakenly believe that car mechanics can refuse to release your car back to you when it is unsafe to drive and you decline the work. False.