This issue comes up a lot
If Calibre had the ability to define the data type of a Custom Column as a
Uniform Resource Identifier then one could could use it to contain the path to Supplementary Material on the local machine, on an FTP Server etc.
The base functionality is already in Calibre, in the ID values and Click to Open items in the Book Details
Sort of surprised Kovid doesn't have it, but maybe its not as simple as all that.
BR