Kamen Rider G was a special Kamen Rider created for then SMAP member's Shingo Katori's SmaSTATION. SMAP members were participating into different things. Goro Inagaki had decided to become a Kamen Rider. This wasn't a parody or something like Akibaranger. He is a official Kamen Rider produced also with Ishimori Staff. Kamen Rider Decade had made a special appearance in this.
2ch began to rumor about the appearance of the actress Yumiko Shaku when she had uploaded a pic from the filming on April 6th. In the pic you can see a prop transporter which says "Kamen Rider" in Japanese.
On April 11th, she had uploaded another filming pic. The scene is a wedding where she is the woman in red trying to stop someone's wedding. She had mentioned she is going to be an evil woman in this. Another Rider Host? She had mentioned that the location of this scene is in Ibaraki.
A user had drilled down the wedding location to be a place called Parks Garden Place in Kasama City, Ibaraki.
Ibaraki Film Commission had announced that there will be Wedding Scene Filming for Kamen Rider Zi-O on April 11. The location of this scene was also Kasama City, Ibaraki Prefecture.
Same location! Same date! Same scene! Ex-Rider Actress!! This can't be a coincidence.
On the same day, a twitter user had reported that he had seen Drive - Kuuga Part Filming near his University. Could this mean that Kamen Rider G part will be in Drive - Kuuga part? Well Kamen Rider G had Terrorist Organization called Shade & Kamen Rider Drive had a Crime Organization called Neo Shade. It wasn't revealed being any connection between the two but...
On April 12th, she had revealed that she had been filming since early morning. Compared to other Drama filming, Kamen Rider Filming begins too early in the morning.
On April 15th, she had posted a pic of a scene where is she is riding the Swan Boat with someone. This is supposed to be the day when filming completed for her. Over there a staff is seen wearing the SPD Jersey which only Tokusatsu Staff have it. This points out that this is the filming of a Tokusatsu Series.
Yumiko Shaku had revealed that the director is Tamura Naoki. He was the one who directed Kamen Rider G. He had directed other Kamen Riders from 555 to Kiva. She had said that she will appear for only two episodes. Two episode appearance will be rare in other Dramas especially for less popular actors. (She might be popular back then but now, no). It could only be possible in Kamen Rider which follows two episode format.
With these many evidences...