Nielsen (http://www.nielsen.com/) are attempting to get their publishing clients to begin to support International Standard Text Codes (http://www.istc-international.org/). As part of that they are interested in getting explicit support for use of ISTCs into as many well-known document schemas as possible. Adding 'istc' as an option for the class attribute of biblioid should do no harm and might be useful if ISTCs are ever to have any chance of success.
That seems entirely reasonable to me.
Added to 5.1.