It doesn't matter where they are - they accelerate the crew/cargo in the opposite direction of the ship's motion AKA they'll increase the crew/cargo's speed relative to the ship.
I was replying to what you said about forcefields increasing the acceleration of the ship itself. I don't think that's possible unless it's an external force.
"And, if you put the force in the direction of the motion of the ship - aka increase the acceleration of the ship - the inertia splattering will also increase" AKA Putting a forcefield in this manner is equivalent - for the crew/cargo - to increasing the acceleration of the ship/to putting stronger engines on the ship.
Seems like a balance is maintained to prevent that from happening. In that sense, the inertia dampers really provide a containment forcefield within the ship that keeps the crew from experiencing any kind of acceleration.
Forcefields - as depicted in trek - are useless in order to negate the effects of acceleration. Gravity is the only phenomenon that may be able to do it - that's becasue it is, essentially, space moving; the objects in a gravity field are stationary in a moving space.
Actually, you can consider many things in Trek to be forcefields of some kind--deflector shields, tractor beams, artificial gravity, both the structural integrity and inertia dampening systems, and even the annular confinement beam used during the transporter process. They all can be considered graviton-based fields that can push, pull, or protect things.
Wizards did it. The inertial dampeners system is one of those things that works on a very loose basis. I think the more you think about it, the less it makes sense, a bit like the universal translator, or the transporter, or the holodeck. Ooh, the comn badges too for that matter.
So you're saying a forcefield in the same direction would act the same as the wall, it would keep the crew moving relative to the ship but they would still be crushed from inertia. But instead of a wall-like forcefield pushing them from behind, it could be something more like a tractor beam that surrounds them and applies the force equally to every part of their body.
Mitigating the inertia problem, the crushing effects of acceleration on the crew, has been an issue sci-fi writers have dealt with since the very beginning of the genre. That's why, in the world's first space opera, E.E. "Doc" Smith created the "inertialess drive" for FTL travel. This is one of the biggest barriers, I think, to real life interstellar travel: without fixing the inertia problem that would leave your crew gooey spots on the back wall, you can never achieve the kind of acceleration needed to get anywhere interesting within a human lifespan. Nick