There are now some KNIME nodes that allow sending messages to a slack channel(s) available from http://update.knime.com/community-contributions/trunk. These will likely progress into the 3.5 build shortly.
Details of the nodes can be found here: https://github.com/webbres/knime_slack
Any problems / feature requests please let me know. Creating an issue in the github repo is most likely to get my attention.