I will try and look out some pictures of the case mad I used the spray on and post them on this thread for you to have a look at.
I should be able to do that tomorrow, then you can have a look at the result.
I have used a mATX Lain Li mother board tray and rear panel for all three of my G5 case mods. I find them to be a less invasive fit. The only down side is that you are left with a tiny exposed space at the bottom where the old extraction fan used to be. To cover this I used and small strip cut from a perforated sheet of aluminum. Once I had spray painted the whole case, back panel, aluminum strip and all, it looked pretty good (or at least I think so. Once I've posted the pics you can decided).
I have only spray painted one of my three case mods and as mentioned it was because of the condition of the case. If you are not looking to spray the whole case I would just leave the back plate as is. The ones I got from Lain Li looked fine with the plain unpainted metal. There are a lot of G5 case mods out there that have done this for you to check out. Another option to help it blend in is to take a Dremel to it with the right attachment and dull the surface of the metal on the rear plate. Once it stops being shiny it should really blend in rather well. If you do that and don't like the finished look you can still always spray it.