I'll try to look. I didn't actually write the integration, it's just a port from an ST integration. I'll add it to my list to see if there is a way to get that. Unfortunately Litter Robot doesn't document anything.
As a note, a small project I'm working on you might be interested in, I plan to shove a contact sensor inside the drawer that when I pull the drawer out for more than 30 seconds (meaning I wasn't just peeking in to see if it was truly full), it will reset the drawer gauge automatically. I don't know why with all the fancy sensors in this thing they didn't do that 
