$base_url and the mystery of broken feed images
Module Pathologic might help your problem, if you haven't tried that. Here is a quote about it (from the module's project page):

... is an input filter which can correct paths in links and images in your Drupal content in situations which would otherwise cause them to “break;” for example, if the URL of the site changes, or the content was moved to a different server. Pathologic can also solve the problem of missing images and broken links in your site’s RSS feeds.
Pathologic is designed to be a simple, set-it-and-forget-it utility. You don't need to enter any special “tags,” path prefixes, or other non-content noise into your content to trigger Pathologic to work; it finds paths it can manage in your content automatically.