We've been using the related-activity element quite differently than is suggested here - we use it to identify related activities within our own programs (e.g. phase 2 of a project, or elements of a bigger projects). We always assumed the other-identifier element would be where we'd be recording the activity number assigned by our implementing organization, thus enabling traceability. We hope to start testing this approach with UNICEF soon.
In our case there wouldn't be a need to identify related activities at the transaction level, as transactions will be almost exclusively with our implementing partner. This could well be different for another donor whose definition of "activity" may be quite different from our - clealy a donor's business model will affect how they need to structure their related-activity data. Which in turn will present interesting challenges when trying to use data from several donors.