In addition to the good advice given already, your original idea of using a replicator might be worth trying (I have not had the chance to try it myself). Make a square tile and cover it with hair, and then use a surface replicator to duplicate the tile over your landscape. This should make it easier on system resources while still giving a good result (hopefully!).
I tried that once and ended up with a lot of obviously flat planes replicated over the terrain. (Still took ages to render too)
It may work better with a surface shaped to always intersect the terrain.
At least it works. I used a 1 foot square plane with 1000 hairs as the grass, around 6 inches high, with some frizz added to provide variation. Surface replicated on a 20 foot plane with variation in rotation and a little scale too, around 650 instances in total. So around 650,000 hairs in total, but handled more efficiently than adding that number of hairs direct to the plane. But yes it is fairly slow to render. But with the numbers involved, it is still fairly impressive that it renders at all!
Sorry, I tried adding the rendered image but something is playing up and it is not letting me…..