To do this you need a PLEX/ someone witha PLEX to invite you

First Normal 21 day invite, the inviter gives the invited person a PLEX.
The invited person uses PLEX to acivate his account (this works, have done so myself)
The inviter now gets a PLEX from the recruiting program.

Result 51 day new account, for no cost at all.

This all does work fine and dandy and have even thought of it myself, but there is going to have to be a lot of trust between both parties for this to work. I think the 21 day + isk program usually works better here on JSP and other forums because it may be more helpful for new players to have that extra isk. That and it doesn't require the recruiter to fork up a plex which he may never get back. You also have to remember the $5 fee to buy the game even if you use a plex.

I agree on the trust issues I suppose, but there is no 5$ fee from what I can tell

There is no fee, and most people know this.
But for the people doing isk for 21 day trials = much better for both parties. Your method had the invitee starting with 0 isk, and the inviter breaks even.

Really the options are 51 days, or 51 days + 300mil ( Pay for subscription )

I would really only recommend it if your opening a new account

the only bonus you can get from the plex way is if you are starting a second/third etc account for yourself. then you get a 51 day account for free. at least thats how i see it.