Nikk
You could try
Combro for the backplate. However they don't answer diving questions (they are not a diving company), so the Zeagle bit may be open to question still.
Looking
here it looks like the Combro should do the trick. Only issue thay may occur is hole positions. If they don't match, your BCD you should be able to find someone who can drill an extra hole for you.
What I don't understand from the diagram, is what happens to the straps on the BCD if you use the harness kit (although you can make your own for about £20)?
Adrian