I think you might be on to something.
I got about a 1/4 of a roll of sod and put it on my porch once when I lived on the second floor in my condo and needed to house train a pup and since a pup has to go out so often I thought I would do this until it got alittle older. I used a spray
bottle of water after every pee and I only had to cut the grass ( with scissors a few times) I used to tell my neighbors "I have to go upstairs and cut the grass"

I worked for me fine and I can't see why your couldn't also just don't put it in anything that will mean Yoji has to jump into it unless he is used to doing that.
Did you try the litter and it didn't work. I use pee pads and they work for me now with my yorkie. All our situations are different and what works for one may not for another. Good Luck