Custom property driving mixRBG - value updates, textures don't

I’m using a custom property on a rig control to mix textures connected to a mixRGB node (BI).

When I slide the mixRGB factor directly, the mix works and the textures update (in material or texture mode).

When using the custom property the factor value updates for each mixRGB node, but the textures will not update in the viewport. There are about 4 mixRGB’s (4 materials, 8 textures) controlled by the custom property.

Is there a setting or something that needs to be set to get the custom property to update the textures in the material shaded mode?