Guy's
I appreciate what you are saying. This topic is certainly simulating the grey matter!
I have a very informative diagram here from RR that illustrates the distribution of thrust generation through a typical turbofan and turbojet. I'm too academically challenged to work out how to post it up at the moment, which is a shame.
The fact of the matter is, thrust will only be produced when a mass flow of gas is worked upon i.e. accelerated. I don't think any of us will argue with that. This manifests itself in different manners, depending on the engine.
As this diagram shows, for a high by-pass turbofan, the lions share of the thrust is generated over the low pressure compressor (the fan). A smaller percentage comes from the hot section nozzle, as the gas is expelled from the turbines.
Turbojets (hence most military a/c) are rather different. Such engines are much lower by-pass ratio, then result being that a greater percentage of thrust is generated at the nozzle. In fact, as this diagram shows, with re-heat engaged, over 90% of thrust originates at the nozzle.
I think where the confusion lies is in the definition of the nozzle of the engine. Jet engine nozzles are a part of the thrust generation process. They are convergent in nature, such that gases are accelerated as they pass through. This is why modern military a/c have variable convergence geometry, allowing thrust generation be tailored to flight regime. This also explains why we can thrust vector.
However, jet engine nozzles have to be rather short by nature of what they do. It is a total non-starter to have lengthy ducts from an engine core to its nozzle - someone try and think of an example anywhere, there aren't any!
The Harrier is a slightly obscure case, but doesn't bend the rules in any way. The variable thrust outlets are the engine nozzles - the front set lie immediately downstream of the cold section (fan) and the rear set immediately after the hot section. It is interesting to note that in the hover, the nozzles are directed forward slightly to offset the action of the large fan in the front of the engine (the Pegasus is more like a turbofan). The reaction controls used in the hover are again nozzles that use bleed air - the thrust is thus generated at the nozzle itself.
Anyway, perhaps I digress. Back to the original point, I hope you can see that ducting exhaust gases from jet engines to a more central point on the a/c axis is impractical. As I stated earlier, there are far more intelligent minds than ours that work in these fields - I think the fact that it hasn't been done illustrates its unfeasibility!
Rgds,
Cuban_8