Wow, this is great. The ehmac.ca forums have a couple of sample AppleScript scripts, one of which worked OK, but they came up with a simpler solution.
You can assign a keyboard shortcut to “Save as PDF…” by going to System Preferences > Keyboard & Mouse > Keyboard Shortcuts. Add a shortcut and write “Save as PDF…” as the menu title. I chose Command-P. It only works if the print menu is already active—so in any program I can hit Command-P twice in a row and get the “Save as PDF…” dialog. I prefer this solution to installing SafariStand, but it’s basically the same amount of work.