Sorry, I thought that you were saying the 20" wheels would solve the gear inch problem and avoid the need for a big chainring if the smallest cog of the cassette is 11t!

There are several Brompton with cassette and the LTwoo Fold B derailleur also on 16" wheels and it seems to work but as written this derailleur is really heavy, big and low quality, I doubt that it will last long.