A new mode for canvas resize

[Spriggana]

I do not know even how to call it, but I cen describe what I'm trying to achieve.

I want to add a borrder to an image. The border should be equal size on every side, but I do not know the dimensions in pixels up front.
So I would like to be able to set something like "5% width", "the-same-as-width-in-pixels height" (and the other way around, percents for height, and copy the pixel value to width).

As I usually work with book/film covers/posters the proportions are not very different so I have created macros with the canvas resize defined as 5% width, 3% height which usually results in more or less equal borders, but an option that would work for non-standard images without tweaking the macros by hand would be very useful.