To my knowledge HAProxy doesn’t support LDAP (or any non-static configuration) backend for authentication.
However, given that HAProxy does have support for Lua based fetches or actions, one could implement a simple web service that interacts with LDAP and exposes an HTTP-based API, and then from Lua one can interrogate this translator service.
(In fact if you search for HAProxy + Lua + LDAP you might be lucky and find something already implemented.)