I don't buy David's argument that a zillion subtypes will be necessary
and will overwhelm IANA. I can see two alternative approaches:
1. Register with IANA all subtypes of application that start with
"application/Davids-Widgets-Inc-". You can then freely use
application/Davids-Widgets-Inc-foobar without further registration.
2. Register a type that has an internally-recognizable subtype system,
e.g. an "application/Davids-Widgets-Inc" in which the first line or so
of the body had further subtype information. This is, in fact, the
approach I am taking with converting the infinitely-extensible Andrew
system, with its literally endless variety of media types, to conform to
XXXX. I've already registered "application/andrew-inset" with IANA, and
all of the Andrew types that don't correspond to an established standard
(e.g. an image format) will be transported under that name.