Rules are applied automatically to incoming mail. Incoming mail has not been read, so there is no need (in the view of the designers!) to be able to use read status as a selection criterion. You can of course select a message or messages and use Message/Apply Rules to run the selected messages through the rules. To operate on a specific mailbox select the message, select all messages (Command-a) and then apply rules (alt-command-l). Note that all rules will be applied, rather than a specific rule as your question would suggest you would like.
You could instead consider using Applescript, which can consider read status or particular mailboxes:
every message of mailbox "Pending" where read status is true
and have your script runnable from the script menu.
AK