Ok, so I looked at the code there and tried to copy it into my app, hoping to get the fit to screen to work. Didn't help.
Then I configured one of my accounts in the stock Email (Galaxy Nexus, 4.2.1):
1 - Wide HTML messages scroll sideways, just like in my app.
2 - I can zoom in, but not out, just like in my app (this worked on Android 2.*).
Android WebView can be configured to "load in overview mode". Sounds simple enough, but the third screenshot shows what happens on the same Galaxy Nexus. The message is fitted to about 2/3 of the available screen width.
So again, I would really love to have this function, but this is one of those things that should take a couple of lines of code, but in practice...