My memory is a bit hazy, but at one point in time an importer company paid the
  large sums (hundreds of thousands I’m sure) to have a certain GT-R model go
  through the safety/emission procedures so they could be imported before they
  were 25 years old. That’s the only case I can think of.

If memory serves, there were a couple people importing R32 Skylines. The one shop stopped doing the necessary work and got caught. Nobody wants to do the work because it's extremely expensive and bureaucratic. The problem with relaxing the rules is that you're either going to accept more pollution or heavily restrict the number of vehicles that can be imported creating an unfair lottery type situation. Keep in mind that other countries, especially Japan, were a lot slower to require pollution controls. IIRC California and Germany were pretty quick to phase out leaded gasoline, but most other countries (e.g. France) were much slower. The safety stuff should be easier to harmonize for imports, but there's still a lot of bullshit in DOT regulations (e.g. everything about US spec headlights).

IMO a step in the right direction would be to carve out some exceptions for EV conversions.