For the Libra Colour, Clara Colour or Clara BW, under developer options, you can enable Enable reading engine capture button. It does put a rather annoying icon on the screen which you tap to do a capture. The screen captures is stored in ./.kobo/reading-engine-captures in a tar.gz file. Opening it and digging down you find a capture.png file. The first attached image is from the power button screenshot to show the icon while the second is from tapping the icon.
You can also with the latest version (v6.0 or newer) of NickelMenu, add a direct screen capture function:
Code:
menu_item :main :Capture Screenshot:nickel_screenshot :capture
You will need to import books to see that screenshot in your library.