Richard,
I don't know if you found my post in your search, but I had the exact same problem and was able to solve it by adding an after-copy shell script.
Create a new text file and put this in it:
Code:
#!/bin/sh
defaults write $4/.Spotlight-V100/_IndexPolicy Policy -int 3
Then click the Options button, Advanced tab, click "Run shell script after copy completes" and choose the file you just made.
Since I did that I've never had any problem with Spotlight indexing my backup drives.