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Perhaps you can contact a local glass cutter to see if they can cut the back for you?

Theoreticall, you could cut the shapton ceramic material with a hacksaw until you hit the glass.
Then you put the saw into the side of the shapton so that you saw between the ceramic and the glass...

Theoretically, the diamond should be able to cut the glass. But aren't those low grit shaptons cheap enough to give it a try?

It's been done before. Try a forum search in the hones section.
Nortons are soft enough to cut with a hacksaw. You'll ruin the blade but they are expendable.
Just don't use water while cutting or...