Ive been working (on something similar) to this in secrecy for months now and
still dont have any joy so I'll be interested to see some of the offered suggestions.

BUT I have been avoiding "chunked" terrains for my own reasons but it may be
worth your time to look into them.
Basically, as I understand(vaguely), a large terrain get split up by the engine
and shown "a bit at a time" as you get near enough. So if you can figure which
bits are being shown, you can then plant entitys and prefabs(?) on it, before
your player gets in sight.
In theory anyway.