Fork Failure on Vintage Steel Tandem Bikes - Any insights?
I'm not saying I don't believe them, but surely they have legal and financial incentives to recommend I err on the safest side. 


Yes, but now that you've asked them and they've instructed you to buy a new fork, if it fails on you and you try to sue them, they'll laugh you out of court. So they've covered that base already.


  you've got to figure that a tandem would have been ridden less over 30 years than a solo


Perhaps, but they recommend replacement after 10 years.


  Just how risky is riding on this 30-year-old steel fork (which seems to be performing well)?


If you read the page linked to above by Daniel R Hicks, you'll see that A) the point of failure is hidden inside the head tube, and B) "Tandem fork failures nearly always cause an accident which puts both riders in an Emergency Room."


  I'm trying not to be an idiot and risk a catastrophy out in the wild.


Then don't rely on equipment that the manufacturer has told you is prone to catastrophic failure.