The ship doesn't have a "North" indicator on it either in Supercruise or normal flight, but the Galaxy map does have a north indicator: the grid reference system. "North" (as defined by the direction towards the galactic Core from Sol) is the direction in which the third of the three numbers is increasing positively.
Note that this is an Absolute North indicator; if you travel 1000 LY East, West, Up or Down and then travel directly North, you'll miss the Core. Your plan of flagging Sag A itself as the "north pole" would have North be in a different direction, depending on where in the Galaxy you were.
Yeah but that's a byzantine system for dedicated navigators. For me it's a question of usefulness vs. strictly polar. A fixed reference point is what I'm talking about. It doesn't have to be Sag A either; that was an example of expedience and colloquially considered the supermassive celestial object at "the center of the galaxy".
The concept of strict nav by magnetic north obviously doesn't really apply in a system where all known points are stored in a navigation computer, and most of that navigation is three-dimensional rather than surface linear. But when you're head down in that dizzying map trying to navigate towards the core from a spiral arm, a quick glance at a glowy carat or line would keep you meandering in the general direction without having to zoom way out, or console yourself that the third coordinate of each gridsquare in your sector was rising as expected.
When you have to deviate from the pathline, say for fuel scooping side trips or to locate rift crossings, it would remain a handy visual reference. You could refer to it when communicating the route to other commanders- "four systems left of the Line."
In short, I think you should be able to set a carat on say Beagle Point if that was your final stop and let the map overlay a bisecting guideline to it while you worked on the actual route zoomed in. Put a carat on the direction of travel end and call it north, 12 o' clock, DoT...whatever you like.
When you're ready to go home you can just set a carat on home and work backwards.
I love that this game overwhelms me enough sometimes to make me wish for stuff and have these conversations.