Hi,
First and foremost, I would like to apologise for not replying to this post sooner - I somehow totally missed it.
You can't set up mipmapping per platform - this would need to be exposed through Unity, and Unity doesn't allow you to do this, I'm afraid. You can, however, disable mipmapping completely, and then use QualitySettings to scale your texture down when you need to. This might work out much better. If you're going to do this, I suggest increasing the amount of padding in the atlas to avoid bleeding across edges.
Its not possible to set cutoffs for mipmap selection, most hardware actually supports this internally to some extent, but this isn't exposed in D3D at least. However, you could increase the Max Anisotropy, which generally improves mipmap filtering somewhat. Its definitely worth a try on your test samples.
Cheers,
unikron