There isn't any definition of what an alternate hierarchy actually means. Its a link to some other object, so why wouldn't any normal URI suffice? In what cases is the semicolon style URI required?

If this is the design for how to integrate Haystack tagging into oBIX, then it won't work well. Haystack takes a design approach that data and metadata is flattened into a simple associative array to model the entity. Therefore everything about the entity would be identified with one URI and object extent in oBIX. Instead the design should address how the notation of marker tags might be represented in the oBIX contract model (and/or extent). Discussion on haystack forum:

http://project-haystack.org/forum/topic/28