There is no reason why you cannot use Diazo in Plone 3, but the integration of plone.app.theming is based on some specific Plone 4 code.
It would not be too difficult to get it working in Plone 3, using collect.xdv as a checkpoint, but someone would have to do back-copying efforts.
The WSGI approach will work, but getting Plone 3 to run on the WSGI stack is quite difficult, and you probably want to avoid a separate proxy process.
Another option is to use the standalone Diazo proxy (see diazo.org) for development, and then use the diazocompiler to generate XSL and deploy to nginx / Apache for production. A bit weirder, but you should work if you don't rely on the specific Plone expressions / variables in your rules file.
Martin
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