I couldn't find an existing bug on this, so I filed a new one. Cross-account virtual folders would allow people to do things like a true global inbox. Patch coming up.
Created attachment 197885 [details] [diff] [review] proposed fix this switches the saved search folder picker to span all accounts, and makes the search backend know how to do searches across different kinds of accounts. Most of the search backend changes are just cleanup of unused stuff; the actual cross-account search stuff is fairly simple - we execute the consecutive scopes of a particular type (online/offline) and switch modes when the scope type changes.
Related to Suite bug 131438 comment 10?
fixed on trunk.
Comment on attachment 197885 [details] [diff] [review] proposed fix it's time. I also ran a testing thread on this on MZ and got very positive feedback.
Fixing Bugzilla Bug 253268 would make this one a lot more friendly, as people are going to want to not have to go find their messages to "unmark" in a different way.