Anywhere, everywhere
Quote from Paraxade:
5. grapple swing - lessee... grapple swing is primarily only needed for grapple ziplines in Elysia, but with the MDB and LSA SWs, the vast majority of those aren't necessary. I think the only place we still need grapple swing is in spire docks, going to and from the aurora unit or to LSA. I can't think of anywhere else we need it. still seems unlikely for now though... but a SW near the aurora unit would theoretically skip this. (I could still be forgetting something, though)
There'd have to be a sw in the aurora unit area and in the boost ball area. You can't cross Spire Dock after getting boost.
EDIT: I also started looking for a screw attack-less valhalla secret world and noticed something. In the room Weapons Cache (where you get the missile expansion by putting in two e-cells), you can get stuck in the ceiling and see out into the open space. It's a bit of a lead.
EDIT 2: Also, if you go in the morph ball tunnel where you get the ship missile expansion backwards, you can go through the wall, at the end, but you can't leave the map. Also, the expansion doesn't exist yet.