Trouble with a complex rewrite

[Wilson Bilkovich]

I am migrating an existing Apache 2.2.4 -> Mongrel configuration to Nginx.
Everything is working fine save for one final piece.

We have a complicated rule that redirects some traffic to a partner
site based on the contents of the query string. At the moment, this is
still a requirement, and I can't do away with it.

The existing Apache rule is:

RewriteCond %{QUERY_STRING} message=.*(%\d+|\b)(foo|bar|baz|qux)(%\d+|\b) [NC]
RewriteRule ^/api/receive
http://target.example.com/horrible/url/?%{QUERY_STRING} [P,L,NC]

The nasty piece here is that "%\d+" in the regexp pattern is
necessary. 'foo' should only match if it is a whole word in the query
string.
(%\d+|\b) allows the pattern to handle either normal word boundaries,
or HTML-encoded entities like %20.
For example:
api/receive?message=foo%20hello should match this rule, but:
api/receive?message=hellofoo should not

Is it possible to duplicate this in nginx?
Thus far I have tried:
1. Making a location {} entry for api/receive, and using if
($query_string ~ pattern)
2. Using if($query_string) inside the / location block
3. Making a location entry for the regexp on its own
4. Testing with simply message=.*foo as the pattern, which makes me
think the problem is more basic than the regexp pattern

None of these seem to work.
I've tried both "~" and "~*" styles, though I haven't actually been
able to figure out what the difference between them is.

Regards,
--Wilson.