You could modify the Chinese font to include the em-space, look for fontforge.
Or you could create a 1-pixel transparent PNG, and wrap it in something like:
<img class="emspace" src="emspace.png" />
img.emspace { width: 1em; display: inline;}
P.S. Actually, it does not have to be 1-pixel, it can be any size, but 1-pixel is smallest. I've attached one. (EDIT: which I see now it's almost impossible to download
).