Scott, no problem taking a spear gun accross the Border but to use it, you need to buy a spearfishing permit from the Martimo office, not sure if their is a representative in Malongane but you can buy one for sure in Ponta da Oura. Can't remember what the last one cost me but it's not expensive. If you ask the local boat operators at the launch area they will point you to the Maritimo's.
If you without a boat it's a bit of a scratch (or a loooong hard swim and walk) spearfishing from the shore in that area.
Bag limit wise you allowed 10 fish in total and whilst it's not illegal to shoot reefies you will get some abuse from the local scuba operators etc if you shoot some.
Also any dark coloured rockcods will simply be itentified as "Garupa" by the maritimo's. Garupa aka Brindles are a no take species in Moz. Would suggest save yourself the drama and just avoid taking reefies in Moz.