I'm not sure if it's even possible on macOS and Linux, you would have to do some research.
The Pd GUI does not load a font file directly (from ttf, otf, etc), and this is good, as things like font caching are good to have, especially when it's handled by the system for you.As mentioned in the fonts/README.txt in the Pd sources, DejaVu Sans Mono is now standard on most platforms (Menlo is a very close replacement on macOS for now) because:* Linux: the font is a dependency, ie. installing pd via apt-get also installs DVSM* Windows: the font file itself is shipped with Pd and loaded into Tk using a custom Tcl extension written in C which calls the Windows API to have the system load the font file and make it available, this way people don't have to manually install DVSM to their system when using Pd* macOS: the font file itself is shipped within the Pd .app bundle and the system reads a key inside the Info.plist which tells it to find and load that file when the Pd .app is opened.
The main patch canvas uses the single font specified by -font-face when rendering. The IEM GUI object labels have 3 font options, but none are settable, ie. replace or add a new option. It would probably be difficult to modify the canvas to work with "rich text" where fonts and styles could be mixed and matched. Maybe I'm wrong on this, but I'm not sure if anyone has tried with the Tk GUI itself, more probably with one of the forks.