no you cant get the randomness but you can program randomness over a period of time. and i agree that theres a lot more to the tape sound than just the warmth but with all things ITB there has to be a little comprimise and a bit of working around the problem to achieve similiar results..
having said that i wouldnt dream of replacing a tape machine with nebula but as a 2nd best its pretty sweet.
you should definitly try the nat4 pro upgrade with the new engine. if you own version 3 then its only 20euro to update it.