I don't think its possible, afaik the only way to update a layered psd file is to open it.
PS now has 'Generate' though which can save out single layers, a combination of layers, and groups to image files, and they update on the fly.
With 2014 you can also specify subfolders too so organising is a lot easier.
If you are using this to output files for use in say 3dsMax its about as near to 'Live' painting on the object as you can get....use an image from the generated psd 'asset' folder in a map slot and it'll auto-update the material.
For example:
[ATTACH]47122[/ATTACH]
With 'Image Assets' turned ON from File > Generate
This will save out back.png, backx2.png (back.png but @ 200% bigger), fore.png, top.jpg(@100% quality), top.png All in an 'assets' folder in the psd source folder PLUS image.png, which is all the layers in that group flattened, in a sub folder called 'images'.
You can see the power of it immediately.
It maybe a case of changing your workflow I'm afraid.
Regards.
MrTom.
