Okay. I've got a good hack to get around this issue until FogCreek implements this in a future version of FogBugz. It involves modifying the source code so it is a no-go for FBOD, but I'll post it anyway for anyone that might find it useful.
For those of you willing to live on the edge, here is how to "fix" your FogBugz installation so that the view-source option under the reply menu for an e-mail will open the original e-mail in whatever mail client you have configured displaying all formatting from RTF or HTML e-mails including images.
Note: In this tweak I didn't go as far as changing the wording on the menu option to reflect the changed behavior. Usually with source code mods I try to change the bare minimum I can get away with to achieve the desired purpose. If you really need to change the label it is in the dlgBugEditing.was file at line 1565 (in my install).

!!!! USE AT YOUR OWN RISK !!!!
Danger! Danger! Danger!
This solution requires editing the source code of FogBugz. The right way to do this under normal circumstances would be to use the extensive
API and plugin framework, but as far as I can tell there isn't an API exposed that makes for a clean way to address this customization. Source code mods, are a last ditch option.
Caveats
- Although the change seems pretty benign. I don't guarantee that this won't break something else in FogBugz.
- Understandibly, Fog Creek might not be willing to fully support (or support at all) hacked versions of their software or issues this may cause.
- Whenever you install upgrades to FB this fix may be overwritten and need to be applied again.
If you have read the above and you are stil willing to accept the potential downsides of running a tweaked FogBugz install, here is how you do it.
Instructions
(1) Backup your FogBugz program directory before you start.
(2) Open the file your-fogbugz-directory\src-Website\download2.was in the text editor of your choice.
(3) Find these lines and modify them as shown below.
'replaced version
'Response.ContentType = "text/plain"
'Response.AddHeader "Content-Disposition", "inline; filename=message.txt"
'new version
Response.ContentType = "message/rfc822"
Response.AddHeader "Content-Disposition", "inline; filename=message.eml"
Note: In my install it was lines 95-96, but it may vary depending on your FB version.
(4) Stop IIS on the machine and the maintenance service.
(5) Rebuild the FogBugz web site.