Sorry to report, it doesn't seem possible at all.

There is actually a scripting feature to set this value in code - I just tried this simple script placed on my terrain:

@script ExecuteInEditMode()
function Update () {
    GetComponent(Terrain).detailObjectDistance = 2000;
}

but it seems that there's some kind of hard-limit which means that even if you set it to a higher value in code, you still don't see them beyond that distance of 250.

On closer inspection, I think this is actually a bug in unity, linked to the way that it seems to make grass disappear in a "perlin noise"-like pattern when the distance is set to 250, but that disappearance-pattern doesn't seem to scale in conjunction with the distance value. I.e. if you set it lower than 250, you begin to get a hard-edge cut-off, rather than the disappearance-pattern moving with your new limit.

Since it doesn't come in closer when you reduce the distance, I'm guessing this pattern also doesn't move out further when you increase the distance, so even though you set the distance value higher, this fixed-distance "pattern" means they remain invisible beyond a set distance.

This seems like a small bug in Unity, so I just bug-reported it! :)