Equivalent of Apache's SetEnv Variable

[Raina Gustafson]

Issue:
I'd like to configure Magento to run in multi-domain mode.
I've been successful doing this via Apache in the past.
It seems that Nginx should be equally capable, but I haven't succeeded.

Server Specs:
Nginx (latest)
PHP 5.3.3
PHP-FPM enabled
Magento (latest)

What I Know:
Apache relies on the SetEnv variable in the virtual host definition or
a similar instruction in an .htaccess file to achieve this
functionality. The specifics are here:
http://www.magentocommerce.com/wiki/multi-store_set_up/multiple-website-setup.

What I Don't Know:
Does Nginx have an equivalent to SetEnv?
Can Nginx be configured to imitate this configuration through rewrites
or some other method?

Larger Community to Benefit:
There are a number of people in the Nginx forum, Magento forum, etc.
asking about this. It would be stellar if someone could provide us
with a definitive answer - even if that answer is 'it can't be done'
or 'proxy Nginx to Apache'. If anyone supplies an answer via the
mailing list, I will make sure that the answer is shared in the forums
and elsewhere for maximum benefit.

Thanks so much!