I really don't see the problem this is solving at a high level of play.
T3 engineers consume 18-20m/s for T4s/nukes/arty, and 24-25m/s (30m/s Scathis) for game enders. So a huge spending of 800m/s requires around 32 engineers in one place to build something meaningful. I don't think 32 engineers is so difficult to manage that we need engineering stations, especially for a one-time thing like an artillery/nuke/game ender rush.
That covers large projects, so now we can look at smaller projects:
Economic structures: Mexes build super fast, so no pathfinding issues there. T3 Pgen/Fab grids are a great option if you have to spend lots of mass (as opposed to T2 fabs), and they average out to 20m/s per engi, which is plenty enough for the time when you will be building them: when you are just building up your eco past T3 mex. And late game when you have a huge eco yet still want to expand it the fab grids you can split into just two or three groups. It'll be safer and is easy to do. Also since eco takes up so much space, you'll need an unbalanced amount of buildrange to make the engi stations good here. PD/Defenses: These things are already very strong and fast to build, removing pathfinding will just make that easy to do on a large scale, which is not a good thing since it buffs a boring and already decent strategy. This also nerfs artillery a massive amount because you can pretty much always near-instantly build a shield anywhere in your base as long as an engineer is nearby. I think artillery is already quite weak and easily countered by proper shielding, so this nerf is unnecessary. Factory assistance: This is one thing where engis can be problematic, because units require a ton of buildpower and engis slowly get into a formation around the factory. It's resolved by the "interrupt pathfinding" hotkey, and you can write a UI mod to run that hotkey when giving an assist order (GAF did this)/the engi comes into range. I personally spam the hotkey all the time when assisting/building so I am biased against engi pathfinding issues ("32 engineers is not hard to manage").I'll also agree with what FTX said on Discord which is that it makes the game into an all-or-nothing situation because there is no way to run the buildpower away to save it (a nuke would be devastating), but it is also much harder to attack buildpower.
As for engine limitations, yes it isn't possible to filter nukes/SMD, and I don't think you can make it so you can only reclaim allied units.
