Hidden Message from a Scholar Inprisoned by the Shogunate

A scholar of western science inprisoned by the shogunate regime sent out poems disgused as a blank sheet. I learned the episode in an epic manga by Minamoto Taro, Fuunji-tachi (Wikipedia, Wide edition, vol.18, p.42).
In the period when a small island in Nagasaki was the only place of contact with the western world for Japan, Takano Choei (高野長英, Wikipedia) studied there under a German physician, Franz von Siebold. In 1839, during the suppression of scholars of western studies in the aftermath of the Morrison Incident (Wikipedia), Takano was inprisoned for criticizing the shogunate's handling of the Incident. In June 1844, he fleed during a fire of the prison.
In January of that year, he had sent a letter to Mogi Kyoichiro (茂木恭一郎), in which a seemingly blank sheet was enclosed (called 獄中角筆詩文 or 獄中爪書). On the sheet are written five Chinese poems, of which the fifth is said to imply his intention of prison break (Choei Takano Memorial Hall (高野長英記念館); a catalogue entry).
Altough it is sometimes called 爪書 (nail writing), it was written with a stylus. The deciphering of the text expressed as indent on the seemingly blank sheet is described by Kobayashi Yoshinori in 小林芳規 (2014), "角筆のひらく文化史 : 見えない文字を読み解く" (岩波書店) (catalogue entry; sample pdf including the deciphered text).
The author is the leading authority in stylus writing. According to his interview (「角筆」の世界　文化功労者の小林芳規名誉教授に聞く), stylus writing (writing by indent) was widely used. The first instance was found in 1961. Many found thereafter were used to indicate reading of Chinese characters. More were found from all over Japan. He further found similar stylus writing in China and Korea. He says his preliminary search also found specimens in the manuscript bible from the 11th to 12th century.