Save As for content retrieved via POST works in most circumstances.

content/public/browser/download_manager.h

Index: content/public/browser/download_manager.h
diff --git a/content/public/browser/download_manager.h b/content/public/browser/download_manager.h
index 947b75547f3edb608973b87a3377ef837d6aa0e4..74b7c6e3b28cda065fc46025a9d4c1d5f29b09d8 100644
--- a/content/public/browser/download_manager.h
+++ b/content/public/browser/download_manager.h
@@ -181,6 +181,7 @@ class CONTENT_EXPORT DownloadManager
                            const GURL& referrer,
                            const std::string& referrer_encoding,
                            bool prefer_cache,
+                           int64 post_id,

[Randy Smith (Not in Mondays), 2012/02/02 19:46:10]

Please document this argument, including the "don't do a POST" value (-1).

(I'm not ecstatic about overloading the POST/Don't-POST signal on top of the
post id, but given our various constraints I think it makes sense.  If we had
DownloadRetrieveInfo, I'd want them separate.)

[cbentzel, 2012/02/02 19:53:06]

Agreed.

[cbentzel, 2012/02/03 15:54:47]

I documented this. I considered adding a constant to indicate "No post" but
couldn't find a great place to do it - plus it doesn't seem like there are many
constants used in content/public. 

[Randy Smith (Not in Mondays), 2012/02/03 19:43:06]

Yeah, I considered asking for a constant but had the same reaction :-}.

                            const DownloadSaveInfo& save_info,
                            content::WebContents* web_contents) = 0;