Hmmm... xmodmap in itself doesn't seem to work for this.
I added the following line to /etc/X11/kb/corgi.xmodmap_us
keycode 37 = Alt_R
And it works as Alt when depressed, however for Alt+Tab combination for cycling between apps it doesn't work, since when I press the key now and hold it, it executes Alt and thus pops the app's menu, but doesn't wait for the Tab, to cycle between apps.
What is the solution?