3 other possibilities I can think of:
1. Add a negative offset to the polygon outline's line symbol so the line draws just inside the polygon - to do that you will need to drill down into the polygon outline's line symbol dialog. This might work best for fairly simple polygons, without sharp kinks or bends.
2. Use Representations - I believe there is an offset option for polygon outlines, which would give you something similar, but with more options to refine it.
3. Create a small buffer just inside the polygons. The buffers will be polygons, but if you make them narrow enough and just set a solid fill they will look like lines.
