For example, I have a scuba diver with an air tank on her back and I need it to move with her spine, but don't want it to twist and deform. Is it just a matter of painting it entirely 1.0 weight or is there a better way? I imagine making a character wearing metal armor you would run into this issue a lot as well.
If the tank is part of a unified character mesh, you can simply set all tank verts to a maximum weight value of 1 to the desired bone. My guess is that this is how @waylow would do it, as he commonly combines my character models into one mesh for rigging.
Alternatively, if your tank is a separate mesh object, you can directly parent that object to a specific bone in your armature.
I prefer a single mesh, but if the costume or accessories will change, it is better as a separate object.
In your scuba tank case, I would have a specific bone for the tank and weight all the verts to that bone. Then use some helper bones and constraints to keep it in place (w/o twisting or deforming etc), but still retain the ability to animate it as well (because you might need to manually fix it for some poses)
It’s a tricky thing to explain how to do via text but that is the general concept.
As for armour and clothing, it’s always a pain the butt to rig. But don’t be afraid to hide the mesh underneath when the character is wearing a costume, and if you look at game models you will actually see all the armour sections deforming in the tricky spots anyway ;)