I have created a search folder called "All" on my IMAP account. I tried to
set it as my startup folder in Outlook 2007 via
Tools->Options->Other->Advanced->Startup in this folder, but it starts up in
Personal Folders\Inbox.

I have also read that outlook supports a /select switch, which could be a
workaround, but I can't figure out the syntax to select Home\All.

The solution was
C:\Program Files\Microsoft Office\Office12\Outlook /select
"outlook://Home/Search Folders/All"

Full details are missing on the Microsoft web site, but are available here
http://www.slipstick.com/outlook/links.htm