Revert the key mapping Caps Lock -> Ctrl. Install Karabiner. Depending on your handedness remap alt_L (or opt_L) to ctrl and use alt_R (or opt_R) as your common alt key.
Remapping in the Karabiner context using the first example below means:
if you enable the radio button for "Option_L to Control_L" the left Option key will act as the left (and only on most portable Macs) Control key if hit and lose its original Option key function. The right Option key won't lose its function and still works as a normal Option key.
or remap cmd_R to ctrl:
If you don't like the remapping of alt/cmd/ctrl/R/L there are plenty of other possible remappings to emulate the CapsLock key. Just enter CapsLock in the search field and choose your preferred remapping:
You may even define your own key mappings (e.g shiftaltcmdC to enable CapsLock and shiftaltcmdR to release it again. These shortcuts mustn't be in use by the system or apps)
(The screenshots are taken running Lion. Since Karabiner doesn't run in Lion I'm using its predecessor KeyRemap4MacBook. The predefined modifications should be the same)