Could you place it outside the run? My birds can put their heads through the wire of their runs to get grass. Maybe that way, they'd only get a tiny bit and prefer the fresh cut grass in the runs. They wouldn't ingest as much, but could perhaps learn it's not tasty?
Interesting. I'd never heard of saltbush and did some googling. Seems it's a common plant that is commercially grown in Australia for livestock forage?