In my experience most pins hae some security device (chain, sprung ball bearing etc.) which may or may not be easy to replicate and your insurers may not be happy if these are missing from some replacement 'bar' even if the bar is strong enough etc.
Demonstrating the metals suitability if required may also be difficult, especially if it has been worked in some way.
I would imagine that sourcing replacement pins from the manufacturer or supplier would be an easy enough task and unlikely to be prohibitively costly.
I do have a man in our machine shop who would likely be able to advise but would need to know pin diameters, loading weights, wall thickness of tubes etc etc etc. I imagine if your talking of machining new pins you may well have someone with similar untapped knowledge.
We use stands from 2-50+ Tonnes and have sourced replacement pins in the past, I have also seen just about everything you can imagine shoved in to hole the adjustment.. M8 bolts are a favourite!
If it was me I would get a few quotes for replacements.

Also forgot to add, it seems you are describing a sudden brittle failure of the steel, in your original post.

Modern steels do not fail in this way, unless some very unusual loading is going on.

Or if the working temperature is very cold to cause a brittle failure - not experienced in normal UK ambient condiions.

Even if you could over load the axle stands to this extent, it is probably more likely the holes in the axle stands would start to elongate or shows signs of distress as the walls of the axle stands are probably the weaker element raher than the pins.