OpenImageIO/oiio — pull request #208 by lgritz (2012-01-05 23:03:47+00:00)
Title: maketx --mipimage lets you specify custom or doctored individual MIP levels.

Like this:

maketx 256.tif --mipimage 128.tif --mipimage 64.tif -o out.tx

This makes the first there mip levels 256.tif, 128.tif, and 64.tif. Levels thereafter will continue to downsample the last one specified (64.tif, in this case).