The explores are not a reliable method of finding Nausicaan ships.
You only get 5 ships (at best 8) per engagement and the chances those will be Nausicaan ships are incredibly low.
The way I understand it, the highest probabity is those missions will involve cookie cutter aliens, the next most likely is Borg or 8472, then you have the chance of one KDF enemy (Klingon, Gorn, Orion, Nausicaan), Terran Empire or Hirogen. And that last batch are the low probability encounters.
A 1 in 6 chance in a low chance occurance pool would probably take ages to achieve.
I guess I'm going to have to wait until Foundry goes live and make a Nausicaan ESC for myself!
I wonder how many you can make, because I'm probably going to have to do several more to finish my ground kill accolades as well.