The trouble with grouting between slabs is you need a reasonably sloppy mix in order to get right down into the gap especially if you are talking about those Indian sandstone pavers with the bevelled edges. If you have a slight overhang can you not push something thin under the edge of the slabs or even a bit of PU foam just to stop whatever you are using from dripping into the water. Once the grout is set it will stay put.

Richard