Sorry, nevermind. Should do my own homework before posting.
Looking at the offending function, "renderResponseBody" in StepKernel,
I notice the comment on line 172:
"// If an action returns Unit, it assumes responsibility for the
response",
which was exactly what I wanted.
Putting () at the bottom of my action works for this.
Maybe some ScalaDoc-type thing for these projects might be nice at
some point.
-Leif