First of all, won't happen. Second, no, you don't need mods to play Paradox games, contrary to what the designers of those mods will have you believe (especially as the games themselves reach maturity much faster than the mods). Third, there is already a public beta test program. It starts when the game is released.
You do need mods. I've been playing Paradox games since EU II, and they require mods. TRP is necessary after a certain point in HOI II, just like Magnus Mundi is necessary for me to get more than a week of enjoyment out of EU III. Crusader Kings wasn't complete until Deus Veult, and Rome was barely worth playing until VV, niether was Victoria before Revolutions+VIP. Now
because Paradox both offers unrivalled depth to its game engines and substantial support I am a loyal fan and avid player of their games, however if they just came out and version 1.0 of each of those games was the final version I would have stopped buying from them a long time ago (along with most other gamers... version 1.0 of a Paradox Game is very buggy). So what I am putting out there is why not just do officially what they are already doing unofficially, but in a manner that isn't going to piss off the casual gamer.
Since the initial release for Paradox is about where Stardock starts with their beta testing for pre-order customers (perhaps a little more advanced, but in some instances not), why not avoid alienating all of those customers who expect a complete product by using those of us who are willing to contribute our time and dedicated enough to pre-order?
Stardock has shown this can work quite well, and I think that since Paradox Games typically come out in such an unfinished state (by nature... they are so very complex for a computer to handle), I at least think that it is something to consider for the future.