Escape can't normally be set to this. However, if you use something like KeyRemap4MacBook, you could set the Alfred hotkey to Cmd+Space and then remap Escape to perform Cmd+Space. You would have to create a custom item in KeyRemap4MacBook to do this but I went ahead and figured out how to do it for you (I've never used KeyRemap4MacBook before, just knew it was capable of doing something like this).
If you installed KR4MB, you would need to go to the Misc & Uninstall tab, click "Open private.xml" and modify it to the following..
<?xml version="1.0"?>
<root>
<item>
<name>Escape to Command Space</name>
<identifier>private.escape_to_command_space</identifier>
<autogen>__KeyToKey__KeyCode::ESCAPE, KeyCode::SPACE, ModifierFlag::COMMAND_L</autogen>
</item>
</root>
Then save and go back to the Change Key tab and click the Reload XML button. The option should show up at the top I believe. Click to enable it, then set Alfred's hotkey to Cmd+Space.
After that, pressing Escape should work fine.