The "read as plain text" feature is global, but everything in your Junk E-mail folder will show in plain text. You could use a rule to move mail there and set a category for those non-junk messages, then use a filtered view to see those messages but not the junk.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers