You're not really putting it in the content stream - you are using the tagged PDF properties to store it "attached" to the content. BUT remember that it doesn't impact display - just such things as searching. Its similar to Alt (as noted in the spec).
It would be interesting to allow Alt for some other elements in the PDF, such as Outline titles, but today that's not there. However, since we are actively working on ISO 32000-2. Now would be a good time to submit a proposal to the committee.