Hi Alex,
I just saw today an ePADD demo that MetaArchive put on, and they addressed some of your questions. Particularly the need: ePADD it seems does a lot as far as processing and discovery/delivery of email archives, but it doesn't provide much at all of the repository functionality that Islandora/Fedora provide. So, I think there is a potential need to integrate the two platforms, at least in terms of workflows.
One concern, though, was the output that ePADD produces from either an .mbox file or a webmail account. The structure of the output (the AIP, in OAIS parlance) has a complex directory structure and includes files that are not human-readable. The output is very specific to how ePADD needs the email to be for the software to run properly. Very non-standard, even compared to a de facto standard like .mbox. So, it's interesting to think about what an ePADD-based email content model would be in Islandora.
Another thing is that ePADD is open source and available on github:
https://github.com/ePADD/epadd. And it's meant to run in a browser, and some parts (the "discovery" module) ultimately meant to run from a web server. Whether or not that translates into potential for being incorporated *within* Islandora, not sure.
Zach