Is there a way to reliably disable mipmapping in the file itself, so that blenderplayer will respect the setting and not interpolate textures?
And no, I'm not making a Minecraft clone.
Mipmaps is a system setting, it would be set by the users startup blend file. Not sure that anyone would recommend overriding a users choice.
To change it you would use python to set
This is related to mipmap settings, you can change in user preferences > systems > mipmap
But this only work inside blender.
In game (standalone) you need a script.
from bge import render render.setMipmapping(0)
related: Python API