Axiege - it's reaction control system.
I think it would be a great thing to see visually and would make your interaction with the movement of your ship feel more organic, which I would imagine is exactly what one would like to feel with the implied level of technology in low-tech ships.
Although implementation could be difficult to do while maintaining compatibility with the modding API and .SHIP files, I do have a theory of how it could be done.
-I do not believe respriting would need to be done. The jets could be comparatively small, say 2x8 in size, such that a reaction thruster would not generally be visible.
-The .SHIP files could use existing engines with new types that modify their behaviour (e.g.; LOW_TECH_REACTION). These new engine types would as previously mentioned respond to WASDQE buttons, and would behave like nodes, in that they would fire omnidirectional jets for the thrust. To ensure the thrusters do not fire into the ship, a check could be made to determine if the transparency level of the neighbouring pixel of the ships' sprite is above a certain amount, where it will otherwise not fire. These new constants for the thrusters would admittedly be inconsistent with how engines are currently used, but would solve compatibility issues. Besides, maneovering jets already use the unused contrail value of engines as a bit of a "cheat", so is that really breaking inconsistency much more?
Now, ther are other flaws with this idea though. Namely, even today we have technology capable of attitude control of spacecraft without thrust-powered systems, such as reaction wheels. Admittedly these have limitations of their own, but with more technological hand-waving you could again say similar systems are built into all vessels. If that were the case, however, why would rotational control be disabled at the same time as engines?
Another issue is with high-tech and possibly midline ships. If you lose the reaction jets on those type of ships, that's some of the feedback gone, and that may be a small detail that puts people off buying high-tech ships as much, even if the difference is only cosmetic.
Yet another issue is determining how to allow reaction systems to be disabled. With the way engines can sometimes be disabled, it seems like it would be problematic to implement the same system for a number of reaction thrusters. This issue further compounds when you consider the previous problem, as low-tech could concievably have the advantage of being able to maneouver while engines but not reaction systems are disabled, while high-tech would not have such an advantage. On the other hand, this could be considered just another quirk which sets apart high-tech from low-tech.