I decided to just go for it and throw it straight on to my Orc quarry tile. I put on a reasonably thick layer, 1-1.5 mm. This is probably the max thickness, in fact I'm pretty sure it will break away after being stomped on a few times with plastic bases, so maybe 1mm depth is the best option. Dry mud would break after prolonged stomping anyway so I'm not too bothered.
I put some extra tufts around the dried puddle, figuring that the places where water would collect - the base of rock formations and natural low points in the landscape - would be where the grass grew.