Here is how I would do it without xmodmap. I'm not sure if I can emulate the SysRQ key this way, but it should work for other keys.

Step One

Install xbindkeys and xte:

sudo apt-get install xbindkeys xautomation 

Now we need to find the bindings of the keys you want to use as Super.
Run xbindkeys -k and press the keys you want. The output should look something like this:

    m:0x15 + c:110
Control+Shift+Mod2 + Home  

(I used Home because I can't use SysRQ on this computer since it requires FN to access it).

Step Two

Create the xbindkeys configuration file:

touch $HOME/.xbindkeysrc  

Now open the file so we can edit it:

gedit .xbindkeysrc

Add this to the bottom of the file:

"xte 'key Super_L'"
 m:0x15 + c:110
Control+Shift+Mod2 + Home  

Save the file.

Restart the computer. The keys you chose earlier should now act as Super when you press them.

Yes, Ctrl+Super+↓ is a Compiz shortcut,:

Open ccsm → Grid plugin (Window Management category) → Binding tab → Restore.