This is another one to cross off the list, tbh. It's an interesting idea, but ultimately not one that warrants the effort it would take to implement.
Make no mistake about it: inverted gravity is something that would be
extremely difficult to crowbar into a codebase as dense and complex as NL/SLX's already is. It would have been far easier if it were a consideration from the very beginning,* but as this has never been a feature in NL, it's not even as if I can pore over old code to see how it might be done.
Anyways, we can already create tunnels and bridges in all directions thanks to the Ladderer and Laserer (not to mention the upcoming Propeller, which is still very likely to go ahead), and we already have
plenty of unchartered territory to be exploring in SLX as it is!
*
Speaking of which, tan_x_dx's WIP engine features multi-gravity from what I've seen, so we may yet have a chance to play around with this idea anyway!