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

content/browser/tab_contents/tab_contents.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/tab_contents/tab_contents.h"
 
 #include <cmath>
 
 #include "base/command_line.h"
 #include "base/metrics/histogram.h"
@@ skipping 916 matching lines @@
          contents_mime_type_ == "text/xml" ||
          contents_mime_type_ == "application/xhtml+xml" ||
          contents_mime_type_ == "text/plain" ||
          contents_mime_type_ == "text/css" ||
          net::IsSupportedJavascriptMimeType(contents_mime_type_.c_str());
 }
 
 void TabContents::OnSavePage() {
   // If we can not save the page, try to download it.
   if (!IsSavable()) {
-    DownloadManager* dlm = GetBrowserContext()->GetDownloadManager();
-    const GURL& current_page_url = GetURL();
-    if (dlm && current_page_url.is_valid()) {
-      DownloadSaveInfo save_info;
-      save_info.prompt_for_save_location = true;
-      dlm->DownloadUrl(current_page_url,
-                       GURL(),
-                       "",
-                       true,  // prefer_cache
-                       save_info,
-                       this);
-      download_stats::RecordDownloadCount(
-          download_stats::INITIATED_BY_SAVE_PACKAGE_FAILURE_COUNT);
-      return;
-    }
+    SaveURL(GetURL(), GURL());
+    download_stats::RecordDownloadCount(
+        download_stats::INITIATED_BY_SAVE_PACKAGE_FAILURE_COUNT);
+    return;
   }
 
   Stop();
 
   // Create the save package and possibly prompt the user for the name to save
   // the page as. The user prompt is an asynchronous operation that runs on
   // another thread.
   save_package_ = new SavePackage(this);
   save_package_->GetSaveInfo();
 }
@@ skipping 456 matching lines @@
 
 void TabContents::OnUpdateZoomLimits(int minimum_percent,
                                      int maximum_percent,
                                      bool remember) {
   minimum_zoom_percent_ = minimum_percent;
   maximum_zoom_percent_ = maximum_percent;
   temporary_zoom_settings_ = !remember;
 }
 
 void TabContents::OnSaveURL(const GURL& url) {
-  DownloadManager* dlm = GetBrowserContext()->GetDownloadManager();
-  DownloadSaveInfo save_info;
-  save_info.prompt_for_save_location = true;
-  dlm->DownloadUrl(url, GetURL(), "", true, save_info, this);
+  SaveURL(url, GetURL());
 }
 
 void TabContents::OnEnumerateDirectory(int request_id,
                                        const FilePath& path) {
   delegate_->EnumerateDirectory(this, request_id, path);
 }
 
 void TabContents::OnJSOutOfMemory() {
   delegate_->JSOutOfMemory(this);
 }
@@ skipping 839 matching lines @@
   }
   is_showing_before_unload_dialog_ = false;
   rvh->JavaScriptDialogClosed(reply_msg, success, user_input);
 }
 
 void TabContents::SetEncoding(const std::string& encoding) {
   encoding_ = content::GetContentClient()->browser()->
       GetCanonicalEncodingNameByAliasName(encoding);
 }
 
+void TabContents::SaveURL(const GURL& url, const GURL& referrer) {
+  DownloadManager* dlm = GetBrowserContext()->GetDownloadManager();
+  if (!dlm)
+    return;
+  int64 post_id = -1;
+  // Check if the URL to save matches the URL of the page itself. One
+  // circumstance where this may not happen is when an embedded instance of the
+  // PDF viewer plugin has the save button clicked.

[jam, 2012/02/03 20:52:42]

we avoid talking about chrome features in content comments. perhaps you can make
this generic and say if a pepper plugin initiates the save?

[cbentzel, 2012/02/04 14:59:08]

Done.

+  if (url == GetURL()) {
+    const NavigationEntry* entry = controller_.GetActiveEntry();
+    if (entry)
+      post_id = entry->GetPostID();
+  }
+  DownloadSaveInfo save_info;
+  save_info.prompt_for_save_location = true;
+  dlm->DownloadUrl(url,
+                   referrer,
+                   "",
+                   true,  // prefer_cache
+                   post_id,
+                   save_info,
+                   this);
+}
+
 void TabContents::CreateViewAndSetSizeForRVH(RenderViewHost* rvh) {
   RenderWidgetHostView* rwh_view = GetView()->CreateViewForWidget(rvh);
   // Can be NULL during tests.
   if (rwh_view)
     rwh_view->SetSize(GetView()->GetContainerSize());
 }