Without any harm I managed to acquire a full load of horse pee, approximately 1 L (1/4 gallon).
My wife witnessed the "happy moment" so the World shall know that there has been no cheating in this experiment.
In front of a camera I poured the liquid over the dung, and behold.. A reddish liquid soon after emerged in the lower bucket.
Now I'll let it wait for a couple of days more to extract any more liquid, before testing the stain out on some sample boards.
At a point, I thought that Roubo might have had to use buckets made out of iron, so that the stain was merely rusty water, so that will still be my excuse if it does not work very well on the sample boards. Theoretically Roubo could also have used ceramics or other types of pottery, but I guess that an iron bucket was the common at that time.
Success is just around the corner.
This will probably never be a popular recipe for a stain.
The level of stain is rising in the lower bucket.