Texture Mipmap Streaming

The docs say that texture mipmap streaming is not supported with terrain because "the terrain system may need full-res textures at any time." And if you're using base maps, I can totally see why that would be the case. But in our case of a large terrain with a single (photo-acquired) diffuse map on it, if we've pushed the base draw distance out beyond the far clip plane anyway, is there any reason why we can't use texture streaming, and what would be needed to achieve this? presumably we'd need to

??either write a custom streamer or
??build a primitive mesh terrain (which we don't actually render) and let that be used for mipmap calculation.

Note that when I say "a terrain", I actually mean "a large grid of terrains, with one texture per terrain."