Here is my first shot at adding that to Pebble: https://github.com/letsencrypt/pebble/compare/master...felixfontein:alternate-chains The number of alternate chains can be configured by setting the PEBBLE_ALTERNATE_ROOTS
environment variable; the default is 0.
I’ll next play around with my client; once I get it work together with the Pebble branch, I’ll create a PR for Pebble.
Update: PR created: https://github.com/letsencrypt/pebble/pull/234