I never tried this but what springs to mind when I read this tried is this...
Before you put the masking tape (I'd use one with quite a stick surface) on the seat paint some lines of reddish brown colour on the seat. When you lay the tape down on these lines and using a new blade on you knife make some slight cuts. Then with your fingers pull the gaps apart, now with some light coloured pastel scrap some dust into the gap to highlight the crack, pat the dust down with a q-tip and the blow any remainder away.
That’s what I’d do. Let us know if this helps, works or is a big failure.
James