If an employer has never requested or reported FAS information in the past, but will do FAS reporting this year, do you agree that the correct approach would be to start with a net obligation/asset for the current reporting year, rather than reconstructing the FAS accounting back to the start of the plan?

Only their auditor can answer this question. But I agree with your presumption.

SoCal is correct that this is a decision to be made by the auditor.
However, I've done this once, when a sponsor was considering going public. The method employed was to use an adoption date ("transistion date") as of 3 years ago, and show three years of financial statements.
Depending on the reason for the transistion, other methods may be valid.