This seems kind of odd to me... why create one set of component coordinates for each res, rather than using relative positions to specify where things go? That seems like it's really the way it ought to be done (IMHO).
In order to use relative positions effectively, you need definitive dimensions for all gauges to be able to place them relative to their centers, top-left, bottom-left, top-right, or bottom-right corners relative to whatever anchors on-screen (the corners or center).
Since FSO doesn't have definitive width and height data for all gauges (Directives, escort, weapons, messaging, etc), I figured the more effective way would be to just to give people more control over the HUD scaling when deciding top-left position.
And really, this isn't odd. Along with relative positioning, a lot of games these days define interface data for three key resolutions (480p, 720p, 1080p) and interpolate for resolutions in between.