There is no official information (except one) about the release of these secret rare cards but there are some Japanese blogs and articles that say how they were released. I will link those here and theorize how these cards were released.
pokemoncard30.com/wordpress-017
This article talks about ‘phantom cards’. Cards that are so rare that not many people have seen them. One of them that were featured is the Secret Rare Rumble cards. It says that after clearing the Pokemon Rumble game, numbers were displayed at the end of the game and have to be entered ‘somewhere’ and it is awarded to 100 people via lottery. Since we know that it is released via the Pokemon Daisuki Club, the most likely situation is that those numbers are to be entered at the Daisuki Club website. Hence being member-only.
kozue.livedoor.biz/archives/50864302.html
This blog is written from a winner of the cards and showed the unopened pack of the secret rare Eevee on the front. The interesting thing here to look at is the date of when the entry was written. August 18th 2009. From this, we know that the lottery entries have closed during that time and the winnings have already been posted out to winners of the lottery. From the start of the game, 16th June OR from the release of the Japanese regular Rumble Cards, 10th July to 18th August, we know that the promotion ran for at most 2 months, but more possibly, 1 month.
The same thing about numbers appearing after clearing the game was also mentioned in this blog entry. My guess is a unique code was released after the game has been cleared and the code is to be entered into the Daisuki Club website to be entered into a lottery drawing.
choco7maron.blog.fc2.com/blog-entry-200.html
This blog entry mentioned the secret rare cards as a Game Clear Prize. Possibly prizes that are awarded to players that cleared the game.
This image on Twitter showed a winning noticed behind the unopened pack. Translation showed that the 3rd line mentioned ‘20 lines of numbers entered…’. This could possibly mean that when the game has been cleared, 20 lines (or digits) of numbers will appear on the screen and players could enter the numbers into the Daisuki Club website and be entered into a lottery drawing.
In short, the promotional period ran for at most 2 months, but most probably one month. In order to be entered into the lottery drawing, one has to be a Pokemon Daisuki Club member and clear the Pokemon Rumble game. 20 lines (or digits) of numbers will then be shown on the screen upon clearing the game and players have to input these numbers into the Daisuki Club website. Once the promotional period ends, the lottery is drawn and 100 sets were given to the lottery winners.
This is the closest we have been to determining the release of these cards. Even though the only official information from Pokemon is from the winning notice, I believe the others are very good and accurate information from different sources.
If anyone has anything else useful to add, please feel free. Thank you.
EDIT : Now we know that these cards were not distributed to random Daisuki Club members. Or maybe not all the sets were distributed via the lottery system and the remaining ones were given out randomly.