LMDB requires multiple features that are not yet implemented on Hurd.

Like BDB/HDB, it requires file region locks. These are used to detect
whether other readers are still alive.

LMDB also requires either shared semaphores or shared mutexes. These are
used for the read and write locks.

In theory, these could be patched out, removing the ability to share an
LMDB instance between processes, and slapd would probably work. I'm
personally reluctant to do that, though, since this would at least
prevent (for example) running slapcat concurrently with slapd, and might
have other implications that I'm not aware of.