Eerder X Rohloff chain tensioner bolted to a derailleur hanger for easy wheel removal, it swivels back just like a rear derailleur. Works with Afline and Rohloff gear hubs, but you need different dropouts than the stock ones that come with your Brompton G line.

This tensioner will be available after I've made enough of these for the dropout Rohloff and Alfine kits. So it wil be later in 2025.