I have also tried the process of selecting all faces:-

I think HemeshGUI creates a new mesh each time the modifier parameters are changed. I haven't tested it, but thats what I would try. Something like:
      myShape = new HE_Mesh(creator).modify(ex.setDistance(distance).setChamfer(chamfer));

Yep. That's basically the route (mesh re-creation) I followed when I wanted to tweak Hemesh settings in realtime. Another route could be to copy the shape on every change, so you would then be able to CTRL-Z. As far as I know, it's not possible to 'undo' modifiers, but I still have much to learn about Hemesh though. One could in theory think of running the inverse of a modifier, but in practice you will then in most cases not end up with the initial shape.

Cheers guys - I was concerned about creating so many object instances on every iteration of draw(), as early experimentation generated some unexpected behavior. 

Hi,

it's a question I get often. Probably because that's how full-blown 3D editors work. But in lightweight hemesh every modifier changes the mesh. There's no history kept (because that would quickly take a lot of memory). The way to go is apply a modifier to a copy of a base object. Interactivity is then achieved by applying the updated modifier to a fresh copy, effectively discarding the previous copy... Just like Amnon did in the terrific HemeshGui.

Newer versions will include a compound object consisting of a base object and a stack of modifiers/subdividors. This doesn't change the approach above but makes it a bit easier...

Cheers,

F.