Putting a substrate (shavings, aubiose, megazorb) on grass will be difficult to get off the grass again, plus the piggies are highly likely to still be able to get to the grass through it.
Something more solid may be needed to stop them having access - a flat panel of wood and then put puppy pads on top of it for absorbency, and then some fleece on top.
Ive just laid some fleece down in my boys run in the past (at the beginning of the season when they can’t have much grass but if was too hot for them to stay in the shed so they had to go somewhere!), making sure it goes under the sides of the run to keep if pinned down. Left a strip without fleece so they could still get some grass but weighed that exposed edge down as much as I could (one of my boys was very determined to get under the fleece to get the grass). Obviously the pee went through the fleece and absorbed into the ground
