I have added the P8  and P8 content runtimes to my list of P7 runtimes (2nd and 3rd in the hierarchy, P7 is first), but when I try to load Alyson into P7 (or any of the new P8 figures), I get severe mesh distortion, so bad that I cannot see or manipulate the figure.

I can 'see' the outline of the normal body shapes, such as 'chest' and the like, but the viewport is just a blur of distorted color, as if the mesh exploded.

Is is just me?

Not likely just you, I would guess that since the rigging advances in P8 could not possibly be supported by P7 your problem is there. I believe SM has stated that the new figures are not backward compatible, I dont see how they possibly could be.

You could perhaps try to import a copy of the Alyson obj into Poser 7 and load one of the P7 rigs into it from the setup room and then adjust the joints.

Many thanks!

 Yeah, I've tried to load Alyson in P7 and the program crashed.

There is no backward compatibility with the new P8 characters.  Rigging is vastly different and cannot be handled by previous Poser versions.