if you (or anyone else for that matter) are still interested in the answer to this, I just tried it for the first time tonight.
The instructions in the release notes should really be fixed, Heres what they say
firstly, the formatting of the arguments ism't very good. the <>'s around the domain are correct, because normally that means you need to put something in place of that. But why are there <>'s around hte zimbraSkinLogoLoginBanner? That part should not change. that is necessary in the commadn. Then there are 's around the path. This normally implies optional parameter, but this is not optional either. While it does need to be replaced, it must be included or zmprov will complain about syntax.
2. To change the logo that displays on the login and splash screens, type
zmprov modifyDomain <domain> <zimbraSkinLogoLoginBanner>[/
It should probably look more like
Anyway, the answer to your problem specifically is something the release notes also don't mention, that your path has to assume /opt/zimbra/jetty/webapps/ as the root. That is, your command should have been
zmprov modifyDomain <domain> zimbraSkinLogoLoginBanner <path to banner>
zmprov modifyDomain <domain> zimbraSkinLogoLoginBanner /zimbra/skins/logos/LoginBanner.png