If you have already changed caps lock to control, adding this to private.xml in KeyRemap4MacBook would make holding control (or caps lock) toggle caps lock:
<autogen>--HoldingKeyToKey-- KeyCode::CONTROL_L, KeyCode::CONTROL_L, KeyCode::VK_NONE, KeyCode::CAPSLOCK, Option::NOREPEAT</autogen>
The threshold for holding keys is 0.2 s by default, but it can be changed in the second tab.
(I suggested changing fn+control (fn+caps lock) to caps lock in the previous question at Ask Different:)
<autogen>--KeyToKey-- KeyCode::CONTROL_L, ModifierFlag::FN, KeyCode::CAPSLOCK</autogen>
If you just want to keep some key as caps lock, you can also change control to caps lock in System Preferences.