Sophie
Good timing on the question as there have been several "sketch" stencils posted at Graffletopia in recent months. Have a look through there.
There seems to be a resurgence in making things more casual. This is a good approach, in my opinion, as people are more apt to provide feedback to things that are not as finished. (If you're curious, have a look at some of Peter Checkland's Soft System Methodology - he strongly recommends hand drawing pictures to get feedback).
I use this regularly.
Unfortunately, OG doesn't allow you to the same line control that, say, Illustrator gives you. Maybe unfortunately isn't the right word. OG doesn't give you this level of control and that makes diagramming fast. I started to create my diagrams in Illustrator to get the look, but then found productivity went down, dramatically for me. So I jumped back to OG -- it's so much better for the type of adjustments you need to make.
Now, there is a compromise to this where you can get the look and the speed. You can create objects in a program like illustrator and import them into OG into a stencil.
The compromise is you loose the editing capability once you copy / paste into OG, but if you are reusing the same objects, that should be OK. The objects come in as single PDF files, so they scale/rotate etc fine. But you cannot modify the lines, colours, etc.
Finally, I recommend looking through Blambot.com for some comic book style fonts -- these are great and much better than overused Comic Sans.
HTH
-- Morgan