Yeah...I don't think the issue of priming and pressure has anything to do with the final gravity. The issue would be as Maylar suggests that the gravity you have is rock solid stable. If you add a known quantity of fermentable sugar (very approximately 1 scant ounce per gallon) and there is active yeast then IF the gravity meant that no further fermentation was taking place absent this addition then the change in pressure would be due only to the gas produced by the added sugars (approximately 2.5 volumes). That said, my imagination is quite limited in that I cannot imagine how you could have a high gravity that was both stable and would allow you to prime by adding sugar: either you have eliminated the yeast (one way or another ) and so no priming could take place or you have not eliminated the yeast and if they can ferment the added sugar then there is nothing to prevent the yeast from fermenting the residual sugars. With beer you are dealing with unfermentable sugars from the grain - sugars that the yeast cannot break down, so if you add fermentable sugar (corn sugar, for example) then the yeast can consume that, but with mead , unless you are talking about a braggot the sugars are fundamentally all accessible to the yeast.
