Uploading to MediaWiki

Well, I am not sure you really need that. I do something similar and at the beginning I was also looking for a way to create the back relation but now I do not thing I need that.

You just need 2 pages, one for "Testcase XYZ" and one for "Requirement 12345".

In your page Testcase XYZ you already listed all requirements associated to you test case. So now you just need in your page "Requirement 12345" to see all Testcases related to your Requirement 12345:

{{#ask:format="table"|[[Category:Test case] |?RequirementID}}

Is this way not a solution for you?

This is only a partial solution, but if you use the Replace Text extension you can globally add categories based on specific text. Of course, the categorisation text would then appear wherever the statistically unlikely phrase was located:

you find, "statistically unlikely phrase"

you replace it with: "statistically unlikely phrase + category"