Shingon and Shugendo are completely different things.Luke wrote: Ven. Eijo always spoke as if Shingon has no connections with Shugendo whatsoever, so I find it interesting to learn that there actually are some.
Perhaps Ven. Eijo can clarify the relationship between Shingon and Shugendo the next time he comes around here.
But its not that they have no connection.
The connection is that Shugendo has borrowed superficially from Shingon and esoteric Tendai. Those borrowings are mainly in the form of practices, and do not extend to doctrine.
Such limited borrowings do not make Shugendo and Shingon strongly connected, or make Shugendo a part of Shingon (I'll ask Jikan to speak for Tendai, if he will).
Historically Shugendo was required to identify with either Shingon or Tendai, but that does not mean that Shingon is in some part Shugendo, or includes it.
In some places Shingon temples also do maintain Shugendo practices, but they are treated entirely separately. Often they are treated as practices not suited for priests, but for laypeople.
(Note that some Japanese Buddhist temples including Shingon temples also have some Shinto practices, also treated separately.)
Also note that while Shugendo has borrowed some things from Shingon and Tendai, most of what it has are original or unique practices and teachings that cannot be found in traditional Shingon (with someone else please speaking for Tendai). Kuji-in is one of those.
Now a question for you or anyone who could care to answer. Why is there so much recurring interest in whether "ninja" practices, the kuji-in, and/or Shugendo are or should be a part of Shingon (or Tendai)?
Of all the questions that could be asked and topics that could be discussed regarding Shingon, this single issue is close to being the only one people seem to be interested in, much to my regret.