So I started looking at solving this with a plugin of my own and I found some good pieces out there:
A: http://code.google.com/p/tiffus/ - A java script library (written i GWT) for parsing TIFF among other files. This would make loading OS-independent and easy to integrate for sure.
B: http://blog.xulforum.org/index.php?post/2011/03/14/Basic-MimeMessage-demo - A small MimeMessage inspector that could be modified to look for inline images and attachments.
C: Time to test things out. Or perhaps a sponsor for the full Thunderbird extension?