To be fair, they had to upgrade to a development release (1.5) of HAProxy to configure it in this way. When we introduced the broxy at Braintree that feature did not exist.
We actually have much the same functionality. We can dynamically route/rate-limit requests on a per-marketplace (or anything else, really) basis, using Nginx's Lua integration capabilities. We may end up writing another post on this, if there's any interest from the community.
(Disclosure - I also work at Braintree)