In our setup we use jailkit allowing only ssh passthrough [1], we have added LDAP support to it (may release the patch later).
Agent forwarding does not provide private keys, but only individual signing requests. This means that while the user is connected, and evil remote can request arbitrary signing, but only as long as the user is connected, and only as long as the users ssh agent is willing to do so. Using ssh-add -c further means that the user will have to accept each signing request. Also note that ssh -W, which is immune to any of these concerns, is fully supported by sshmux.
Correct me if I'm wrong, but "Jailkit" does not seem to stop an evil legitimate user from poking around other hosts with forwarding and such (which they can as long as ssh -W functions, within the limits of what a firewall allows the jump host to do in general). sshmux allows you to lock users to targets, rather than lock general capabilities of the entire jump host. This is because, while I provide clients access to various hosts, I do not trust them enough to have any unnecessary privileges.