As earlier reported in EASI passport domains support in PIC / LCS 2005 SP1, EASI passports (Custom domains in MSN/Live) has actually been supported in LCS 2005 Public Instant Messaging Connectivity since October 11th 2005, even though the KB article KB897567 "Known issues that occur with public instant messaging after you install Office Live Communications Server Service Pack 1" said that it didn't.
Last I checked (April 2nd 2007) the information still wrongly stated that it wasn't supported, but it has now been corrected (I have reported it several times both through feedback forms and my contacts in US) as follows -
As of October 11, 2005, all EASI Windows Live ID domains are fully supported for the Public IM Connectivity (PIC) service. This means that Microsoft Office Communicator users can communicate with MSN users by using EASI Windows Live IDs through PIC. EASI ID enables MSN users to use the domain name of their choice to create an IM address through Windows Live ID. For example a user’s EASI Windows Live ID could have been firstname.lastname@example.org
This was a great service for MSN users. However, the service caused problems when PIC-enabled users tried to communicate with MSN users by using Office Communicator. With the change of October 11, 2005, you can use the EASI Windows Live ID with PIC by making a simple change when you add a user to your Communicator contact list.
A Communicator user who wants to add the EASI Windows Live ID of email@example.com would add the following address to the search field: username(contoso.com)@msn.com
This increases the effectiveness of the PIC service because contacts that have unsupported EASI Windows Live ID domains no longer need to change their domains in order to use the PIC Service.Remember that if you implement LCS and OCS and SIP with same namespace, as your users has used earlier in EASI passports (E.g. firstname.lastname@example.org), then the msnusers will be required to change their passports after the PIC service has been activated ... and this is by no means a "smooth process". Read more about this in Issues with EASI passport domains, LCS and MSN Messenger.