hey,
One solution I use is doubling textures using photo editing software.
Many of my shaders use the alpha channels of textures as secondary textures.
You can make the alpha channel of the bump map a specular map for example.

if you do this you can basically get up to 32 single-channel textures that you can parse out for your own needs.

You could also add extra samplers from shared file space.

In other words, your task is a cake-walk in shader performance terms.