The font uploading system was built to support the font use on Cards. And this is working pretty well.
The concept of tapping into the app-embedded fonts for HTML articles wasn't part of the original engineering -- and at the moment this isn't supported. I suggest dialing-back the enthusiasm just a bit for now. Sure, express how much you'd like to see this, but please understand that any hack on this may be unstable or break in the future.
Product Management understands the appeal of having HTML access to the embedded fonts, and they and engineering are going to take a look at this. As soon as it's viable, Bob and I will share the methods.
Sorry for asking you to dial-back on this, but I think in both the short-term and long-term we'll have less frustration this way.