I'm not using an apple keyboard and don't have a Left ⌘. Can I change the "Host Key" to be the Right ⌘? How?
The GUI setting is under Preferences -> Input -> Virtual Machine:
This sets the host key to the right command key (VirtualBox 4.3.10):
VBoxManage setextradata global GUI/Input/HostKeyCombination 54
The command will accept any value, but VirtualBox will not start if you supply an invalid value. 55 is the standard left command key, 61 is the right alt key. I found the values via educated guesses (aka "playing around").
There is a bug that appeared in VirtualBox 4.3.8 (maybe earlier?) that prevents the Virtualbox “Host Key” from being changed. It got fixed in VirtualBox 4.3.14. If you are using a VirtualBox version that is affected by the bug, here is a workaround:
After changing the host key, select some other configurable shortcut (no need to change it), and then click OK.
You could try adding this to private.xml in KeyRemap4MacBook:
<autogen>--KeyToKey-- KeyCode::COMMAND_R, KeyCode::COMMAND_L</autogen>
It might not work if you don't have a physical keyboard with a left command though.