Carbon fibre makes sense as that is what ILM ended up resorting to using for their super huge models to prevent obvious sagging. So I could see it used here as well. But it would indeed be expensive. A structural engineer could also come up with some kind of load bearing armature as well (something that the nacelle and pylon are built around to handle the weight, as opposed to the pylon itself being the load bearing structure). But hey, even with studio models, when they are stored the nacelles are braced to help prevent sagging anyway. So I see no problem with Steve's low tech solution.
Bi-plane rigging on a model sags over time, same as hard joints if things aren't balanced. Plus, one would need to drill some holes in spots to mount the thread or wires anyway. So I don't recommend that method for bracing a model. I've tried it on smaller models and even then the results were dodgy at best.
Personally, I think the primary reason why Matt Jefferies went with a swept back pylon on the Phase II E design (which evolved into the movie E) is because the straight pylons probably caused a few headaches with the building team when it came to fabrication and use in effects shots. Plus he was likely thinking ahead that even if Phase 2 was short lived as a series, the model might need to be around for awhile in case it was used in future projects as well (or hung from the ceiling in the Smithsonian).