This is a very hard question, and only a specialised lawyer will be able to give you an answer. I will just address the technical aspect of your question.
Technical solution of your problem
The easiest technical solution to your problem is to use another patch format. The patch and diff utilities support several formats, and an especially interesting one for you is the format generated by
Output an ed script.
An ed script is a small programm deleting lines in a file and inserting some other lines, much like what the nowadays usual unified patch format describes. However unlike the latter, ed scripts do not contain contextual information. Technically the ed script contains only your work, so you may see it as an adequate way to distribute your patches. It seems that git-diff does not support the production of patches as ed scripts, so you will need to manually produce them, the easiest way to do so is probably to diff two directories containing the original and the modified version of the source code.
As a side note, I am asking why not bring the original code owner in the discussion? Has the original code owner any good reasons to refuse you to distribute patches? There is chances that you can reach an agreement on how to distribute patches on this code, as it seems to me that you add value to the existing software at no cost for the product owner and without eating their bread.