Anyone know or have any thoughts on whether there was ever consideration given to granting vanity calls based on ham radio seniority, or even if there was -- does the FCC lack too much info on old licenses?
Well if you go back to the 70s, there was still in place a system that let Extras request 1x2 calls. Starting in 1968 (according to AC6V's web site), FCC would (upon request) issue a 1x2 call to an Extra who had been licensed for at least 25 years. At the time, that was the only way to get a 1x2--FCC wasn't issuing them routinely when someone upgraded. But it really wasn't a vanity program--you couldn't select your callsign, you had to accept whatever 1x2 they issued you.
In 1976, FCC changed this program in two ways. One change was that they started reducing the 25-year requirement in increments. First was anyone who met the 25-year requirement, then those who had upgraded to Extra prior to Incentive licensing, then those who had upgraded before the NPRM was released, then those who upgraded before the R&O was issued, and then eventually any Extra could apply. Each of these groups was given a three-month period before the next group was allowed to apply. Second, they allowed applicants to submit a list of callsigns. so this really was a precursor to the vanity program.
This program continued through the end of 1977, but was abruptly terminated when an FCC employee was accused of rearranging the processing order of applications in return for cash payments.
I don't recall whether there was any such discussion when the current vanity program started, but I agree with N4KZ that I don't think anything like that will return.
As for records, FCC has microfiche that go back to 1968 or so, and computerized (ULS) records back to the late 90s. Earlier paper/microfiche records were destroyed when the basement of the FCC office flooded. My speculation is that this was from Hurricane Agnes in 1972--check out the figure here
showing 15+ inches of rain in central PA.