I've come up with an AppleScript that I launch with Butler (that's optional, of course) that puts the path of the frontmose Finder window into the clipboard. That way when I'm saving a document, at the point at which the save dialog comes up, I can then run the script via Butler, and hit Shift-Command-G to bring up the "Go to" dialog. Then I just paste the clipboard and get the path I need to save where I want.
I've included a commented-out piece that perhaps a more savvy person can get to work. This would basically eliminate some of these last keystrokes. The problem seems to be related to the fact that "Go to" belongs to the Finder or maybe the OS, and although it may be summoned in apparently in any application's save dialog, it chokes when calling it via System Events -- presumably because the front app won't own Shift-Command-G, but that it is rather called by the OS somehow.
[robg adds: While I don't know the answer to the AppleScript question, you can sort of work around it with Butler.
Addendum: Peter Maurer, Butler's author, has provided the one-keystroke solution in the comments. Thanks, Peter!]

