Volume 273 of the Journal of Shinto Studies (神道宗教 — note that the official English title is not a translation of the Japanese) included an article by Suzuki Akifusa (鈴木顕房) on “The Relationship between the Ōmiwa Clan and the Miwayama Ritual”. Miwayama (三輪山), Mt Miwa, is a small mountain in Nara Prefecture that is very important for the early history of Shinto. It is currently the site of Ōmiwa Jinja (大神神社), and there are a number of early archaeological sites, which are among the earliest evidence for practices that can confidently be identified as Shinto. (See my essay on The Early History of Shinto for more details on this issue. Affiliate link. In case you somehow thought I had no financial interest in selling my own books.)
The question at issue in the article is that of who performed the rituals on Mt Miwa. We know that the Ōmiwa clan had primary responsibility for them in the early eighth century, when the Kojiki and Nihonshoki were written down, because those texts include stories explaining why they have that position. However, there are reasons to think that this may not have been true for the earlier period, and this article looked at the evidence again to come up with a plausible account.
The problem is that the evidence is extremely limited. There are the records in the Kojiki and Nihonshoki, and there are the results of archaeological work at Mt Miwa, and that is it. Fortunately, although the Nihonshoki was written down in the eighth century, some parts, covering roughly the fifth century onwards, are thought to be closely based on older records. (We know that Japan had writing in this period, because there are surviving artefacts with writing on them.) Some of these were modified to fit later ideas of what the history of the Imperial line should look like, but it also seems that some things were left as they were, when they were not seen as presenting a problem.
Thanks to this, we have about half a dozen references to rituals for the kami on Mt Miwa and to the Ōmiwa clan that probably date back to the fifth and sixth centuries. The first references involve Yūryaku Tennō, in the late fifth century. Yūryaku Tennō didn’t call himself that — he predates the use of the title “Tennō”, and “Yūryaku” was a name made up for him in the eighth century — but he is the earliest one whom pretty much everyone agrees really existed. We have references that seem to be to him in Chinese records, and in inscriptions on contemporary objects that have been found in tombs. In the references in the Nihonshoki, Yūryaku Tennō is involved in venerating the kami of Mt Miwa, but does so indirectly, by instructing other people to do so. However, these people do not appear to be members of the Ōmiwa clan. Archaeological findings suggest that the rites at Mt Miwa were performed at several locations on the mountain at this period, and Suzuki combines this information to argue that the rites were the responsibility of people local to the mountain, but not to a single, well-defined clan.
In the sixth century, on the other hand, we have clear references to the Ōmiwa clan, and one of these references associates them with Mt Miwa, but in this case there are no explicit references tying them to the rites. On the other hand, the archaeological evidence suggests that the rites became concentrated in an area that was later closely associated with the Ōmiwa clan, and that by the end of the sixth century the clan had built a large house near the ritual sites, which is said to have been turned into a Buddhist temple by a member of the clan in the eighth century. The archaeology is not consistent with the details of the story, but does support the general idea, because the site of the house seems to have been turned into a temple within a couple of decades of the death of that clan member. (The story was recorded centuries later.)
Based on this, and the records in the Kojiki and Nihonshoki, Suzuki argues that the Tennō never directly venerated the kami of Mt Miwa, but that the intermediaries changed. In the fifth century, it was simply people who lived in the area, with no particular lineage specified. During the sixth century, however, the lineage that became known as the Ōmiwa clan took control of the rites, and in the seventh century, and into the early eighth, they were clearly the priests of the kami of the mountain.
If this is right, then the tension between lineage-based and locality-based qualifications for venerating a particular kami go back right to the beginnings of Shinto, and so it is hardly surprising that we still see both at work in the contemporary idea of an “ujiko”.