All it requires are a few files:
Credit goes to the Sensation forums/ROMs and tons of PM questions to random people :P
The flashable zip below puts the first two files where they need to be, but the build.prop edit needs to be done manually, as everyone has a different build.prop and replacing yours with mine will undoubtedly change things your ROM chef has put into their own build.prop.
Use root explorer or your file explorer of choice and add this line in your build.prop:
ro.htc.framework.screencapture = true
I recommend adding it somewhere at the bottom, but be careful; your build.prop needs a blank line at the very end.
Once the zip is flashed, and the line is added into build.prop, a final reboot will make everything work (unless you do it all in one go).
Press and hold the power button followed by the home capacitive button. You will then hear a camera snapshot sound, and a toast message will follow. Snapshots can be found in JPEG form in the camera shots folder in your gallery.