Hi,
Sorry but, no, single-user apps are single-user apps and do not get extended/modified by the Yunohost community, AFAIK.
the only “hacks” about app is creating install/update scripts for them to integrate nicely with Yunohost (SSO, admin interface, etc).
it depends on the application. Transmission, for example does not read any ldap entry, it is just protected by SSOwat and nginx’s reverse-proxy. Apps that can read LDAP like owncloud, for example are configured to use the local Slapd server and that’s it. I don’t think anybody would hack an application to change its authentication method, maybe I’m wrong, but it seems tricky higly time-consuming and hazardous.