HKTraveler,
You are right in saying that Invitation members of Marco Polo Club receive the same benefits as Diamond card members but the only difference is that Invitation is based on lifetime basis as Diamond is on an annual basis as you mentioned.
From what I believe, to get an Invitation membership to Marco Polo Club, the management of Cathay Pacific (either the Chairman or CEO) has to invite you to be a member. The VIP's of HK Govt, CEO of multi-nationals are offered Invitation Membership.
Basically, it is similar of Qantas' Chairman's Lounge program where the CEO invites a member of be a member of the program. Apparently, if you want to be considered for Chairman's Lounge, you need to send in your resume to CEO's office and Geoff Dixon will make the decision wheather you get the membership or not. I am not sure if CX operates in this fashion.
If anyone knows more about Invitation membership of Marco Polo Club, we would like to hear from you.
Cheers,
oneworld Fan.