Let me try to explain our challenge here.
We have a traditional version published originally in 1938 but it is still in use. We will keep this historical version still available in print but also through the emerging Bible app.
The printed copies of this particular version has up until today used a layout where the cross references appear directly after each verse. [I do not know how one could add an image here, so if you would like to see what this has meant in practice, let me know]. Thus, originally when we created the PT files for this version, no xo marker were used, because we saw no use for them. We could also download the version into DBL without any trouble, but that was some years ago.
In the app something similar will happen: when you click the verse text, the app will show you the cross references as hyperlinks.
In my understanding, we do not need the xo marker at all in this particular case, since the user of a printed copy or the app will not face any challenge in knowing into which verse the cross reference in question happens to refer to. The problem would, as far as I can think about it, emerge only when cross reference are separated from the text and placed e.g. on a column beside the text or as foot notes at the bottom of a page. This would, of course, then happen only with a printed version of the Bible.
I do not think we should press the users to employ markers (and items) in PT when these will not be used while publishing the text. It does not seem to me a good practice to make people add things to their texts and later make them remove these when the time comes to publish the Bible. I would rater make the use of such markers as optional. In case we already have a wonderfully working system to add the xo marker when the users need them. This is why it is difficult for me to understand why these always must be used.