Some bunnies do like to pee on soft bedding. Jack would at times. VIvian doesn’t but she likes to move it around and chew holes in her blankets. Like Roberta suggested — untreated sea grass mats are good, but they are rather flat so I am not sure how soft they are (Though there may be some thicker one that I am not aware of). There are Timothy based mats which are thickish. But with the grass mats, if they are peed on, it would not be as easy to clean.
Though I don’t have to worry about my bunny peeing on bedding, I have had to in the past with other bunnies, and so I just go to a fabric store, get several yards of soft fleece like material and cut them to make a few soft beds. If they get peed on one day, I just put it in the wash, and put a new one down the next day.
I have also used the foam mats and they work well for bunnies who don’t chew on them, but you would have to be very careful to make sure your bunny doesn’t eat it.
Note:Bunnies who are on flat surfaces and even on carpet can get sore hocks. There are some factors that may contribute: Bunnies who are overweight, whose nails have grown too long(putting pressure on the hocks), who is stuck sitting in their own urine/poo for too long, older/sedentary, or are of a certain breed (like Rexes with thin fur on their feet), are more susceptible.