1. Right-click inside the material room panel, then "Select all" from the pop-up menu.
2. Right-click again, "Apply to all..."

You can create MAT poses with P6, by the way. Once you have all the materials set up, save to the Material library as a material collection. Then open the .mc6 file in a text editor ...

The beginning of a Poser 6 .mc6 material collection looks like this:

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Change it to look like this, instead:

<b></b>

Save the edited file to one of your pose folders, with a .pz2 extension.