Correct me if I'm wrong, but I believe asteroid macroporosity is meant to represent the amount (usually a percentage of its total volume) of void space between separated or semi-attached pieces of asteroid's body, what we consider a single mass body but might in fact be a combination of several larger chunks of materials held together by their microgravity and low radial velocity (axial rotation), or finer grained materials, be it on its body or formed between the cracks at the time of the impact (not necessarily a spectacular one, mind you) of one chunk of the asteroid into another.
That's why the word rubble is used so frequently in such documents. If these individual chunks would be somehow embedded into one another, I guess the word conglomerate would be more frequently used, but in geology, conglomerates are not necessarily highly porous, or not at all.
But to rather leave this description to astrogeologists that study the geology of celestial bodies, here's an excerpt from the document on Asteroid rubble piles: How big are the pieces? (PDF) prepared by the Department of Geological Sciences, University of Tennessee, and Vatican Observatory, of the Vatican City State for their 62nd Annual Meteoritical Society Meeting:
Introduction: Many lines of evidence lead to the conclusion that
asteroids are strengthless bodies, piles of rubble held together only
by their own self gravity. Among the indications leading to this
conclusion are the observations that all asteroids spin slowly enough
to be held together by their own self gravity; that near-Earth
asteroids have odd shapes matching those predicted for strengthless
bodies undergoing tidal distortion; and that the densities of
asteroids are significantly lower than corresponding meteorite bulk
densities.
This latter work allows one to put numerical estimates
on the amount of macroporosity in asteroids. If S-type asteroids like
Ida and Eros with a density on the order of 2.6 g/cm3 are made up of
material similar to that in ordinary chondrites, then they must be at
least 30% empty space. Given the 10% porosity typical of ordinary
chondrites, these asteroids must have void spaces on a scale
comparable to the meteorites representing another 20% of the volume of
the asteroid. For C type objects like Mathilde, Phobos, and Deimos,
the observed low density (1.5 g/cm3) implies a macroporosity of at
least 20% if they are made of rocky material similar to low density
hydrated carbonaceous chondrites. However, these dark asteroids are
observed to be anhydrous; to make them out of water-free dark
meteoritic material would require that they were 50% or more void
space
I believe this Google-fu find describes well enough what macroporosity in asteroids is. So, sadly no Swiss cheese, or even grains of instant coffee. Just piles of rubble held together by various forces, and this macroporosity varying greatly from one celestial object to another. It is a percentage of void space between cracks, separate pieces held together by their own gravity and not perfectly matching with their joining sides, rubble and dust on their surface, etc. I.e. a measure of how solid/hollow an asteroid is, or its bulk, large-scale porosity — macroporosity.