The whole alias name feature seems to be independent of namespaces. I even remember this independence as one of the arguments for using aliases to not care about namespaces.

But there are parts of the definition that require the use of a namespace.

• BrowseName of the alias nodes
• AliasName in AliasNameDataType is a QualifiedName

This creates small and major problems
(minor) What namepsace does a server use if server does not care about namespaces for aliases?
(major)
The AliasNameDataType works with ExpandedNodeId and therefore it is not necessary to repeat the namespaces of the referenced servers in the namespace table of the alias provider (server). The server just need to provide the list of referenced servers in the ServerTable.
But if there is an aggregation, there are two major problems
(1) Merging Aliases from different aggregated servers will not work since they use 'different' namespaces
(2) The aggregating server need to repeat all namespace from the aggregated servers and needs to do a namespace mapping

One option to solve this problem is to define a standard namespace like "http://opcfoundation.org/UA/Aliases" that is used as default namespace for all alias nodes.