Thanks Wolfgang,
The problem with NextCloud, Box, OwnCloud, Google Drive is that their local copies of files are not accessible using the apparently relative file paths that you might expect, and I don't have a systematic way to translate the relative path that you might use identify a file via a wiki link into an actual local file location for the aforementioned cloud file providers. You can see the actual paths of individual files if you peak at the file information in the Files App or in Quine, which will give you a clue as to why it is hard. Dropbox had the same problem for the longest time, and only recently moved to a more "conventional" hierarchic way of accessing their local file copies. So unless the aforementioned providers follow suit then there is really nothing I can do to solve this, so far at least.
Sorry :-(
Chris.