SectorDamage seemed like the right solution, since it can be applied to the Transfer_Heights destination sector and will damage the player when they're ""underwater"", but that's still a no-go since the rising lava means that I need to apply the tag to sectors that aren't yet submerged -- SectorDamage then kills players who come in contact with the ground. Welp.

Hence this suggestion: a SectorDamage flag to disable damage via ground contact, effectively restricting damage to ""underwater""-only. Would much prefer this (or some other sane engine feature) to hacks or zillion-things-insanity.