Earlier this week, EBSA rolled out a "new and improved" website. Just came across this page:
https://www.dol.gov/agencies/ebsa/employers-and-advisers/plan-administration-and-compliance/reporting-and-filing/form-5500
-- and remembered this question. Forms, schedules and instructions going back to 1995 -- not quite all the way to 1990, but maybe an easier interface than the IRS for those years.