Technique for extreme depth of field in macro photography

Sounds like large format tilt to me...

Using tilt only creates the "toy world effect" because the technique creates a thin sliver of focus w/in the FOV, which appears as a very shallow depth of field; which creates the impression that the scene must have been very close and therefore very small.

It's a mental/optical illusion; and when tilt is applied to close-up photography in the more traditional manner it can have the opposite effect (unusually large DoF).

The only other method of achieving extreme DoF is using the hyperfocal distance with a short focal length lens; but that couldn't include macro level magnification.

Assuming you don't help the focusing plane with a tilt lens (which is a major tool when your objects don't have too much height above the plane), the essential thing is to have an entrance pupil that is small against the subject size. That will cause diffraction that tends to be the lesser evil in macro photography than a limited depth of field. For that you want to use the smallest aperture available with your lens. Note that tele extenders placed between lens mount and (macro) lens will not just increase magnification but also increase aperture number. They allow you to go further in the tradeoff between depth of field and diffraction than the lens designers thought reasonable. With a pinhole aperture and a comparatively large focal length, you'll need a whole lot of light (and/or long exposure times).

Also these days you have a chance of reducing diffraction effects using something called "deconvolution sharpening" since the diffraction effects are essentially independent from the distance of the objects in the scene. Effective deconvolution amplifies noise significantly, so you want to work with the lowest ISO values your sensor supports, again meaning lots and lots of light.

For high-depth-of-focus photography, you can deal with some problems at the expense of others by simply omitting the lens completely and only working with a pinhole aperture and very high exposure times. Not having aperture blades at all will avoid "sunstars" (albeit the geometry of macro lense apertures should already try to minimise them, but a variable aperture is harder to perfect in that regard).

If aperture and the projected size of the subject on the image plane are held constant, a longer focal length will provide more depth of field than shorter focal lengths. For example at f16 and 1:1 macro, a 200mm lens will provide greater depth of field than a 50mm lens.

For deeper depth of field in a macro image, use a longer focal length lens.

For conventional lenses, longer focal length lens will require proportionally greater extension (via tubes or bellows) than a shorter lens.

This may explain what you saw...a long focal length lens to increase depth of field with an extension tube making it even longer...for example a 500mm lens with an extension tube to achieve macro projection on the image plane would be approximately a meter in length.

That's why those old 8x10 view cameras often had such extreme bellows extension capability.