Logically thinking..
1) You are not changing any LOOKS of zimbra so you don’t worry about displaying anything in your Application Login..
When you do pre-auth the splash comes (with zimbra logo) which is OK and then when user is inside the zimbra app the zimbra logo is shown in top..which is OK
So i don’t see any reason why you will need to put zimbra logo in YOUR APP from which you using pre-auth to connect to zimbra.
..unless anyone at zimbra differs
2) Open Source community always happy to take good stuff
Raj