If you are willing to carry on with this, perhaps we can capitalize on your efforts and incorporate the changes into calibre.
It would need to be a per-library preference (dbprefs) for the character. There is no need at this point to build the GUI to set it. Instead, set the default to something other than period, change all the right places to use the preference instead of the character, and then test it (tag browser display & edit, main gui, content server, searching, template functions, and who-knows what else.
).
Once it is working, we can change the default back to a period and add the GUI to change the preference. The tag browser preferences panel would be a good place. I would be willing to build this part. I already know how & where, and that stuff can be very confusing.
If you create a repository on launchpad, we could exchange source and be sure that we don't lose anything. When done, I would merge the changes into an up-to-date repo and submit to Kovid. Or you could, if you wish.
Do note that Kovid has an unofficial "you touched it last" bug fix policy. He would expect you to fix any bugs related to the changes. This may be more work than you are willing to sign up to do.