Polygon outlines pen attributes: use -W together with -Gp?

Dear List,

I have an arbitrary closed polygon occasionally way larger than my plot. I'd like to indicate the inside side of the polygon without filling the whole area. My idea is to have a solid outline (regular -W option) and add a fill pattern of specified width to the inside side of the outline. 

I read through the Cookbook carefully, especially chapter 9.13 "Specifying pen attributes" and 9.14 "Specifying area fill attributes", but there seems to be no "official" way of achieving this. 

I thought about shrinking the polygon a little bit and fill the developing  gap via -Gp. However, this would lead to a variable width pattern due to projection distortion. 

Looking into the -t option (PDF transparency), I thought about replacing the pattern with a solid line of large width and some added transparency to achieve a similar result. So far I was unable to confine the line to the inside of the polygon. 

Any ideas are highly welcome. 

All the best,
Kristof

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