I just recently played with ground fills and found the results to be pretty bad. I documented the problems, or issues, I encountered here.
I read thru the posted github bug report posted by @fai and I didn’t notice any mentions of the issues I found in my next post here. The issue I mention there is after I was done with the ground fill in pcb view, I couldn’t select some components in schematic or breadboard views. When I hovered the mouse over the components, the tool tip that appeared by the pointer indicated the part was a copper fill and not the component itself. I think there is more than one issue with ground fills…
I think you should be able to select any net as a seed for the ground fill. Ground fill doesn’t have to be a ground connection. In fritzing, copper fills have no connections, they are just copper areas that are not removed during pcb manufacture. Ground fills do have connections and can be connected to anything really. Use a ground fill with the +Vdc as the seed, and it could connect all the +Vdc connections together. Should be able to create one side of pcb as ground plane, and the other side as the +Vdc plane.
Or at least, I hope you could do that. I haven’t tested it…
Randy