Calabash safe mode? As a web service?

I was wanting to run Calabash behind a web service that has the pipelines
generated and fed into it from a Javascript pipeline GUI tool.  For this, I
would want to restrict pipelines from writing to the filesystem (other than
temp), and running arbitrary commands.
I can't seem to get calabash -S (safe mode) to run on anything, though; it
just gives a dynamic error about access to that resource not allowed on
simple pipelines like say, the included pipeline.xpl test pipeline.
Is there more info about what calabash -S safe mode is supposed to do, or an
example pipeline case that it works with?

(Now that I think about it, I suppose I could just take things like p:exec,
p:store (and p:http-request file:// method=POST ?) out of the GUI.)
Thanks,
-Leif Warner