I think the reason is tied to the Gerber flash code legacy, where 500 mils (12.7mm) was the largest physical aperture available. I don't know if there is a workaround or update available to that, so that's what we have to deal with.
There is no way to add decals to inner layers, so that is not a solution. But you can add multiple copper shapes with association, so one decal will do. Well, one decal for the 4 layer design, and another for the 8 layer design. The trick is in dealing with the inner layers. You will need to build the decals with the exact same layer structure as the PCB. Now, just make a copper shape on each of the layers, and associate them one at a time. If you need to use thermal reliefs, it needs to be built in to the copper.
The whole point is that my customer wants to be able to use the same part for each type of board design (2, 4, 8 layer designs) without having to worry about the decal. When I am done, they will take over the maintenance of these layouts, and they do not want to become (as they put it) PCB design experts. Having the capability to assign a Copper Shape to "Inner Layers" just like you can for pad stacks would solve this issue.
My answer was based on using a PADS layer structure that matches the finished PCB, as I make use of the Layer Thickness properties. If you don't need that, then using the same 8 layer stackup, with custom CAM outputs for each design, will work fine with a single decal.