I have installed the XShell Plugin for jenkins and use the following command to start nightwatch:
cd "C:\path_to_testdir"
nightwatch
The only thing I'm currently stuck on is the fact that nightwatch starts and runs the selenium server, find the test it needs to run, but then hangs at starting up the browser...
EDIT
I found out why it looked like Nightwatch was hanging here. Services apparently get their own hidden desktop. The solution to that is to not run Jenkins as a service. Just start it using java -jar {directoryOfJenkinsFile}/jenkins.war