I sent feedback to OmniGroup requesting that they add jitter/squiggle line stroke styles to the next release of OmniGraffle. Omni responded that they will add this request to an existing request for hand-drawn effects, but that feature requests usually make it to the product development team when there is enough clamoring for the feature. So if this is something you're interested in, send them feedback via their support page (scroll to bottom of page for email address.

Here's the feature request I sent:

I've been hearing a lot of people asking
for some way to make drawings created in OmniGraffle appear to be have
an unfinished look by simulating hand-drawn lines. A lot of people get
by doing squiggly lines in CAD or other drawing programs. But we've
also seen people use Visio to do this.

What would be ideal for Graffle is if we could have a stroke option
that simulates squiggly or jitter line effects in a future release.
Please consider this a vote from the many of us who are asking how to
do this on the design mailing lists.