I am not a site moderator, but I would like to make a case for these being included as fair game, based on how in the Japanese market they are considered an extension of the manga/anime market.
When a manga-based musical comes out, information about it will be printed in the manga magazine itself and often as a book cover band for the manga tankouban (graphic novels). A small sign with a photo from the musical can be displayed next to the tankouban in Japanese bookstores.
In the case of Sera Myu (the Sailor Moon musicals), some of the theatre programs, CDs, and other collectibles have featured original, exclusive anime cel artwork, indicating the creators' desire to associate their stage production with the anime series rather than only with the original manga (some examples: 1 2 3 4). The kigurumi (a.k.a. anime-gao) live-action stage shows (scroll down to「52 更新 200３年 ６月 ８日」) also feature the anime cel artwork for their merchandising.
According the the Asahi Shimbun newpaper, the Nihon 2・5jigen Musical Kyoukai (Japan 2・5-Dimension Musical Society) was created earlier this year in order to attempt to raise the quality of anime musicals as part of globalization, based on the acknowledgement that anime and manga has garnered fans around the world.
If anime musicals are included as on-topic here, drama CDs and novels based on manga would also need to be included. If Japanese fans relate these mediums in their minds and associate these expressions as part of the same franchise, then it does not make sense to me to try to erect walls between them as if they are not related enough. For example, if it is not off-topic to ask questions about anime merchandise, anything produced by/for the anime musicals would also fit under that umbrella (especially the ones featuring anime and/or manga artwork). For the musicals' collectibles to be on-topic but the content of the musicals to be off-topic seems problematic for consistency.
Video games adapted from anime/manga and/or using a Japanese animation art style could be included as on-topic but would not necessarily need to be as a result, since video gaming is a somewhat different market in Japan (manga and video games are not sold in the same department in Japanese stores which stock both), whereas a photobook produced from the stage musical, a novel adaption, and a drama CD could be sold in the manga section of a bookstore.
Live-action TV series and movies derived from manga could also be included as on-topic, as they can be advertised by the manga publisher in the printed works as well as in the bookstore; however, Japanese do not tend to associate these as strongly in their minds with the original work, perhaps due to the celebrity actors who they are already well-familiar with overshadowing the source of the material. Live-action TV series are not considered shounen, shoujo, seinen, josei, etc. as the CD dramas and novels adapted from manga are.
In other words, allowed anime musicals, audio dramas based on manga/anime, and manga-based novels as on-topic would not necessarily force opening up the whole of Japanese pop culture to also needing to become on-topic.