I wouldn't necessarily disagree with that assessment, SJ. It is why we welded cross-hatching on dozer blades, tines and the like on our properties way back .... It not only re-built the "cutting edge", it also hardened it very considerably compared with the base metal. The base metal has the greater tensile strength and toughness; the welded repairs had greater hardness.
Where I have drilled is on the inside of the link (obviously ... ), and the weld is around the outside of each link, so about 8-10 mm away from where I have drilled. I reckon that this should be outside of the "heat treated" zone, at a rough guess.
However, that end of the chain now 'complies'. There is nothing that says that it can't be welded as well as bolted ... Anyway, as I keep saying, the trailer is a pre-September 1989 build, and this is evidenced by the "R" number stamped on the draw bar, so it doesn't have to comply with any of this stuff. However, I also see the sense in these things, and want it to be as safe as possible, consistent with reality ...