From the Kircher Society blog, here is the story of the self-mummying suicidal monks:
Scattered throughout Northern Japan are two dozen mummified Japanese monks known as Sokushinbutsu. Followers of Shugendô, an ancient form of Buddhism, the monks died in the ultimate act of self-denial.
For three years the priests would eat a special diet consisting only of nuts and seeds, while taking part in a regimen of rigorous physical activity that stripped them of their body fat. They then ate only bark and roots for another three years and began drinking a poisonous tea made from the sap of the Urushi tree, normally used to lacquer bowls. This caused vomiting and a rapid loss of bodily fluids, and most importantly, it killed off any maggots that might cause the body to decay after death. Finally, a self-mummifying monk would lock himself in a stone tomb barely larger than his body, where he would not move from the lotus position. His only connection to the outside world was an air tube and a bell. Each day he rang a bell to let those outside know that he was still alive. When the bell stopped ringing, the tube was removed and the tomb sealed.
Link: Kircher Society | Daruma Forum - Thanks The Good Reverend!
not much there, just the word "allegedly"
http://en.wikipedia.org/wiki/Sokushinbutsu
"Mystery of the Tibetan Mummy"
Siberian Buddhist Mummy
Buddhist Mummies of Japan
They are Buddhists - they don't believe in your creator.
Of course, popular referance is not conclusive evidence, but at least it is less likely that it is something that was made up by westerners.
I recently found out that the first English-language book on the subject is being published soon. (It's about time someone translated some info. about these guys!) This is a link to the text: http://www.mcfarlandpub.com/book-2.php?id=978-0-7864-4880-7