Yes, basically you would have to define a custom URL scheme, just as there is one for clicking on the format links. But this is not something I want to encourage. Once this facility is present then people will start demanding a convenient way to create the links, that the links be automatically preserved when copying between libraries, a way to link between entries in different libraries, a convenient way to manage the links and see what books link to a particular book, a way to edit them in bulk, exporting them in catalogs, browsing them int the content server and on and on.
Basically, this would be opening the door to adding citation management to calibre, and I definitely do not want to go there at this time. And if I did, hacking comment like fields to allow inter book linking is not the right way to do it.
You are, of course, free to make these modifications in your local copy of calibre
