My first thought was that you could make a Finder alias, but I see that the Finder doesn’t allow aliases to folders within packages.
Another possibility would be to create a link in Terminal:
ln -s path/to/original path/to/link
For “path/to/original” you can just drag the Site folder onto the Terminal after typing “ln -s”
For “path/to/link” you could type something like:
“~/Desktop/Site” (without quotes)
This should create a link on your Desktop.
I don’t know if Transmit will accept this.
It’s possible that we should replace “Show Site Folder” with “Export Site Folder” (within Xway). This would export the internal folder to an external location. It also has the advantage of not opening up the document package (making accidental user mistakes less likely).