I agree, I think "invitation only" is a marketing ploy. If you inquire a few times, and have a decent expenditure with a Platinum Amex that has had 0 payment issues, I am sure they will take you.
If not, this concerns me as a shareholder of AXP. If there are people willing to fork over $7,500+ so they (and their significant others) can have a 45 cent piece of titanium, and they're not taking advantage of that, I have an issue with that.
As a former Centurion who is more than happy with my Plat, I could care less about obtaining Centurion again. It just isn't worth it. Without Platinum Starwood, Hyatt Diamond, and higher tiers of airline status, not much incentive.