Split bolt is a regular way to join 680.26 conductors. As long as it is listed for direct burial and copper I don't see an issue. It would be pretty expensive to do one on every corner of a 12" grid tho.
The Erico folks say they make it prewelded and ready to go. I looked at the catalog and didn't see a 3' wide mat. They seem to be 4'
If you are really buying some I imagine the salesman would be happy to talk to you.
