I had some dwarf sag growing in a tank and planted some crypts in pots to keep them seperate. The dwarf sag got into the pots anyway. I find it won't grow on a rock and so I use rocks in the places I don't want it. If nothing else the rocks make it easier to cut and prune the plants that grow under and beyond them. I imagine the partition would work well if you keep up the pruning. It just needs to be solid all the way down to the glass.
As for sand and dirt, I find the root mass is so thick that in the end it really doesn't matter. For pruning individual runners I find it is just a matter of being gentle and maybe leaving the filter off for a while to let things settle. Scissors and long tweezers help a lot.
I have also read that one should not grow sag and val together though that was many years ago. Innes claimed they were incompatable but that was a long time ago. I have never tried.