I've played with various combinations of diffuse and specular nodes in Poser to try and find a balanced solution to this, but always the hard line of the terminator remains, particularly with side lighting when IDL  is used. I think the problem is we end up needing both front and rear scattering effects to deal with the terminator.
BB's previously suggested way to partially soften the line away from the shadow region using a blender controlled by a smoothstepped diffuse node also falls down when using global lighting - the harsh terminator line is tere again.
You can't force any light past the terminator in a diffuse manner, but you also don't seem to be able to diffuse the transition in the other direction, away from the shadow region.

The only material based way I've found to even slightly soften the terminator is to disturb the object surface using bump or displacement, but this offers only a very slight diffusion and doesn't always produce desirable effects.
Using light arrays also offers a partial solution but sharply increases render times.

I keep hoping I'm missing something, but we seem to need a new material node to even fake this.

 

PoserPro2014(Sr4), Win7 x64, display units set to inches.

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