How do I specify a different signature for when I reply as opposed to
composing a fresh/new email?
No direct way of doing it, unfortunately. It should be possible to write a
script that checks once per minute (the quickest cycle time that can be set
in schedules) at the front window, determines if it is a message composition
window, then determines if the subject contains 'Re: ' and sets the sig
accordingly.
If you think this would be useful, give a shout and I'll make one up for
you.
You can send feedback to the developers on this issue. Use the last menu
item under the 'Help' menu - "Send Feedback on Entourage". This message will
go directly to the developers. The more people that request this feature the
more likely it is to be implemented.