Good question and one I deal with a lot too being just short of 6-8. I value status for many reasons and one big one is access to "comfy" seating. An aisle would be a must for you I think. I would suggest checking in early and simply asking the GA if they might find an exit row or bulkhead. I am always amazed at the short people with no status who somehow end up sitting on exit rows beside me. Amazing. You can ask for an upgrade but you better have one convincing smile!

Try calling ahead too a day or two in advance. Same with the counter agent when you check in. For some reason I find counter agents in Europe, FRA especially, are willing to help me out when I'm in a bad seat. Through the years I've even been upgraded twice for reasons I only know to be unmerited favor with the person in front of the screen.

I did ATL-JNB twice last year and though ATL-SVO is old hat to me I was beside myself at about 12 hours in Coach. Indeed, getting to know the FAs helps pass the time.

Remember the loo has a lot of legroom too! No upgrade needed even.

Not sure what a doctor's note will accomplish. Being nice and asking directly might be your better course of action.

Good luck!