This post is the third in a short series about the Imperial succession crisis and the Shinto establishment’s attitude. You might want to read the posts about the nature of the crisis, and why the Shinto establishment is opposed to female, and female-line, Tennō, first.
The Shinto establishment’s proposed solution to the crisis is to restore (some of) the male-line male descendants of the former miyakë to the Imperial family, possibly by adoption, so that they can succeed as Tennō.
Historically, the Japanese establishment was aware of this potential problem with the succession, and maintained a number of “hereditary prince” miyakë. These were families that, although they were descended in the male line from a Tennō, did not, normally, become Tennō, but remained in the Imperial family and thus were available to provide a new male-line Tennō if necessary. It sometimes was. As I understand it, branches of these families were removed from the Imperial family from time to time to keep the numbers down, and so, when they were all removed from the Imperial family in 1947, that was entirely in keeping with tradition. At that time, the current Tennō (Shōwa Tennō) was still relatively young, and had three brothers and two sons, so no-one was particularly worried.
This solution is popular with hardly anyone else, because pretty much no-one outside the Shinto establishment knows who these people are, and it would be very difficult to get public acceptance for replacing Prince Hisahito’s (putative future) daughter with one of them.
More than that, I do not think it is coherent. That is, I think it is just as large a rejection of tradition as having female-descended Tennō.
To the best of my knowledge, there is no precedent for this. There is an ancient precedent saying that male-line descendants of a Tennō retain their imperial status for seven generations (if I recall correctly — I am not going to chase down the exact reference for a blog post), and there is a case of a Tennō who was born outside the Imperial family, but restored when his father (the son of a Tennō) was returned to the Imperial family to become Tennō. However, the most recent Tennō in the ancestry of the former miyakë reigned in the fourteenth century, and there have been about 25 generations since then, which is way over the historical limit. There have also been several generations of non-Imperial members in the families since 1947. There is no precedent at all for bringing people into the Imperial family under those conditions, and no reason to limit it to the former miyakë if you decide to break tradition. There are other families with well-attested male-line descent from a Tennō in the distant past. You could have a vote.
Japan is in a position where, if Prince Hisahito does not have any sons, it must either change the historical tradition of Imperial succession in one way or another, or see the Imperial line go extinct. The overwhelming majority of people in Japan want to change the tradition to allow female line inheritance; the Shinto establishment wants to change the tradition to allow non-Imperial distant descendants of a Tennō to join the Imperial family. A few people want to see the whole tradition die out.
This is one issue where I cannot see the Shinto establishment winning. Their arguments are weak, and they are in a tiny minority. Given how important it seems to be to them, this could be a major blow.