I was actually wondering this myself the other day, and since I couldn't
find anything, I worked out a workaround myself. It's a bit kludgy, but it
will more or less behave the way Mail Act-On does with Apple Mail in that it
can be used to trigger/apply a rule.
What I first did was setup a "Stop Rule" in Entourage's rules for each of my
account types. The condition is "All Messages" with no actions other than
the checkbox to NOT apply any further rules. This is the same in concept to
the stop rule that Mail Act-On creates, essentially preventing any rules
below it from being triggered automatically.
Once you've done this, create your additional rules for filing messages,
flagging them, running scripts, or wahtever, *below* that Stop Rule. They
will not be processed automatically because the Stop Rule prevents it, but
they can still be triggered from the "Apply Rule" menu item (found on the
"Message" menu or the right-click context menu).
Make sure the names for these are unique from any other menu items, as
you'll need that uniqueness to assign keyboard shortcuts in Mac OS X. Once
you have them named and setup, go into your Mac OS X System Preferences,
select keyboard, and create a set of new keyboard shortcuts specifically for
the "Microsoft Entourage" application. Give the shortcuts the same name as
your rules, and these key sequences can then be used to trigger those rules.
You should also see the shortcuts appear beside the individual rules on the
"Apply Rule" sub-menu.
I haven't yet found an easy way to have one key sequence trigger *multiple*
rules, like you can in Mail Act-On, but thus far I haven't found that to be
a signficiant limitation.
