I think it's more to do with manufacturing and inventory management. With the invitation system they can smooth-out demand and prevent peaks until manufacturing is well underway.

Of course one of the ways in which it smooths demand is to turn people away; I was interested in a Meizu Ubuntu phone until I learned I had to 'compete' for the privilege of giving them my money.

Same with the original OPO, by the time I won an invitation ( after waiting several months ) I had gone and bought an LG G3.

The opposite. To little supply for the demand. They used the invites to "throttle" the purchase rate to be able to keep up. They probably thought an invite system was a better aproach than a weekly free-for-all aka. SuiciDDOS against the webshop. Hopefully, this time supply will be significantly higher, meaning the invite system will be removed as soon as they know they can keep up with demand.

Even if that only takes six months the phones isn't going to be interesting by the time you can actively purchase it.

You buy phones for their "interesting" factor rather than actually wanting to use it as a phone? If so, you're probably an extremely tiny minority in that regard...

By not interesting I mean a better version from some other company will be available.