Until today, I was completely unaware that it's possible to install custom fonts on iOS, even though this is officially documented by Apple. In a nutshell, it works pretty much like installing custom homescreen shortcuts, i.e. via configuration profiles that contain the TTF file's data as a payload.
This is also a nice use case for Pythonista's Open in... menu integration. You can select this script in Pythonista's settings as the handler for opening files from other apps. Then just download a TTF file with Safari, and open it in Pythonista, which will automatically launch the script to install the font (the script gets the file path as a command-line argument).
If you launch the script directly, without using the Open in... menu integration, you'll simply be asked for the URL of a TTF file you'd like to install.
The script creates a configuration profile (basically just an xml file) with the font data, and lets Safari download it by starting a local HTTP server.
After installing a custom font, you can just use it in a lot of apps (e.g. the iWork suite). Before you ask: Editorial does not support custom fonts at the moment.