Per Lou's comment on TEI-L:
it seems that tagUsage is only useful when describing use of an element, but if you want to describe more complex usage, you need to use an ODD. However, I am having trouble finding this stated explicitly in the Guidelines.
The introduction to section 2.3.4 (#HD57) implies that more detailed information might be elsewhere in the header, and section 18.104.22.168 (#HD57-2) does not mention any other location where you might put such information.
I might also expect to find a mention of this use of an ODD in section 1.2 (#STIN) and section 23.3 (#MD). I confess to not having time right now to re-read these sections closely, so I might have missed a reference. But I would also expect this use to be stated somewhere obvious, near the beginning of these sections.