Reddit mentions of Samurai Zen

The sound of one hand clapping was a koan made by Hakuin, he thought it was superior to Choshu's Mu koan. It's a bit false to think koans are set in stone from anecdotes about old masters. During the Samurai era, monks were known to make up impromptu koans on the spot..

You're probably equating koan with the well known blue cliff records and such, but koan does not have to restrict itself to those.

For me the most unusual would be the koan 56 of this book: https://www.amazon.com/Samurai-Zen-Warrior-Trevor-Leggett/dp/0415284651 :

"
In the seventh year of Ko¯an (1284) there was a great drought.
In every region the rice-fields and farmlands dried up and
there was no sign of anything growing. The Vice-regent
(Ho¯jo¯ Sadatoki) anticipated that such a bad year might cause
disturbances in some areas, and he asked the great Zen master Mugaku (Bukko¯) to pray for rain according to the traditional ceremony (once) used by Zen master Eisai. He gave
orders in the capital that in front of the stone torii of the
Tsurugaoka Hachiman shrine at Kamakura an altar twelve
foot square should be erected of pure sand, and arrangements
made for the ceremony with its accessories of rice-wine and
so on.

Bukko¯’s attendant disciple Isshin (the editor of the
Records of Bukko¯) did not at all welcome this performance of
a rite of the Shingon mantra school, as Eisai, though professing Zen,
had done. (Imai’s note: It is said that the rite which Zen
Master Eisai performed when he prayed for rain at Kamakura in
June 1201 was a ceremony of the Shingon sect with which Eisai had
once been connected.)
Bukko¯ said to him: ‘When you go to a village, follow the
village ways. What is wrong with that?’
The attendant, when he saw that the Master was going to
do it, hastened away first, and when he got to the altar
jumped up on it and said:
‘Today instead of the Master let this novice make the
prayer for rain. The Zen way of rain-making is an unusual
one. Do Your Honours please look,’ and he briskly tucked up
his robe, spread his legs wide, stuck out the ‘one-eyed dragon’
and made water on the altar.

At this Sasaki Sukemori, the official in charge of the ceremony,
was aghast and angry. He arrested the disciple and was
taking him under escort to the Kita-mandokoro police headquarters,
when on the way suddenly a great downpour fell,
bathing the road. Sasaki realized that there had been a divine
meaning in Isshin’s action of making water, formally thanked
him with warmth and set him free.

Tests

(1) Where is the rain-god? Say!

(2) What virtue was there in Isshin’s action? Say!

(3) If there is virtue in making water on an altar, then this
instant try making water on this Buddha-altar to test
the virtue. Make the proof of it!

(4) The teacher lifts his nyo-i stick and says, ‘This thing,
and Isshin’s one-eyed dragon – are they really in the
end the same or different? Say!’

(5) If you really understand, try manifesting great action
immediately: make proof of it!

(Imai’s note: Many koji (laymen) taking this test have tried imitating the action of making water, and received a slap on the face
from the teacher for it. Don’t imitate them!)
This became a koan in Kamakura Zen at the interviews of
Ko¯an, the 14th teacher at Enkakuji.