Best way to deal with blend shapes?
 

If I have a character which is a single poly object, what is the best way to use facial blend shapes and have everything below the neck ignored?
I know (at least I'm pretty sure) that it can be done if you use a wrap deformer with your high-res model and have the head seperate on your low-res, but I'm wondering if there's a less annoying way? (I've had all sorts of things go wrong with wraps).
Idealy I'd like my lowres/highres relationship just to be a smooth proxy node on the low-res.

Can anyone please suggest the best workflow here?

Any help would be very much appreciated...
Thanks.