This. The entire GNOME stack is based on a steroided-out C-based OO type system in the same vein as the OP's PDF... and it's actually how it gets bindings to every language ever so easily...
Anyway, I was sent down a rabbit hole of wanting to do object-oriented C for a language independent core as well, starting off with buying the physical copy of this PDF, because I liked it so much...
I have to say GStreamer is probably the absolute best, most ambitious, shining example of such a type system, as it's already built upon GObject, fixes a lot of its shortcomings, and just really takes the whole thing to the next level with a complex, highly OO architecture. Fantastic reference and inspiration for this kind of stuff.
Anyway, I was sent down a rabbit hole of wanting to do object-oriented C for a language independent core as well, starting off with buying the physical copy of this PDF, because I liked it so much...
I have to say GStreamer is probably the absolute best, most ambitious, shining example of such a type system, as it's already built upon GObject, fixes a lot of its shortcomings, and just really takes the whole thing to the next level with a complex, highly OO architecture. Fantastic reference and inspiration for this kind of stuff.