There are a few screenshots that I took of my GNOME Shell that I was able to hack to include RGBA attached to this thread.
Here's how I was able to do it:
I edited /usr/share/gnome-shell/theme/gnome-shell.css (with gedit). In the "#panel" block, located in lines 73-79, I added, before the closing curly brace,
background-color: rgba (50, 100, 50, 100);
After this, I restarted GS, and this is what I got:
Next, I edited (with gedit) the block where the calendar popup was (near the very end of the file). In the code, it is "#calendarPopup". Just before the closing curly brace, I put:
background-color: rgba (0, 0, 0, 0.9);
And this was my final result:
Notice in this second screenshot, the calendar popup is also transparent, instead of black.
Here is what a RGBA-hacked GS looks like with a window open: