Well, I made some tests and even if took some time to report them here, once again, I found that Calibre is already handling this gracefully:
In the application settings folder, ie. either "Calibre Portable\Calibre Settings" on a portable instance or the equivalent one on regular installation ("%USERPROFILE%\AppData\Roaming\calibre"), just look for a "fonts" folder (or create one if needed).
Then, simply put your font files directly into it since sub-folder don't seem to be handled there, leading to something like:
Code:
Calibre Settings
+-- fonts
+-- my_font_a.ttf
+-- my_font-Bold.ttf
+-- my_font-BoldItalic.ttf
+-- my_font-Italic.ttf
+-- ...
At Calibre's e-book viewer start, the locally copied fonts are made available and may be selected in the Preferences/General/Font options like the system-wide default ones.