Yeah, even a supposedly simple vehicle like an EB is still an entire
world of small little details to know about and deal with. Whether it's the engine or chassis or wiring, it's the little things that you need to know ahead of time so that they don't come back to bite you.
That metal buildup and machining idea is probably the best route, but not sure they can do it with the bearings in place? Maybe so, but never had it done myself, so not sure.
A common alternative is a Redi-Sleeve or Speedi-Sleeve that fits right over the axle and (carefully!) presses on to that spot to create a new surface.
They're very thin, very smooth on the outer surface (literally a brand new polished sealing surface, and are made precisely for this type of situation.
Not sure how long they are, but I'm betting you can get a set that would fit outside the bearing area and only effect the seal surface. Plenty of members here have used them, so hopefully someone with direct axle experience will help.
My only experience with them was back in the early seventies when I was trying to save a harmonic damper. After I decided that I could not install it myself with my limited tools, and a shop would charge just enough to make the replacement part less expense and trouble, I just went ahead and bought a new damper.
So my own installation experience is obviously very limited.
Good luck. But definitely check into both methods if that groove is as deep as you say.
Remember too though, to check to see how much leeway you have in seating the new seal. If you have a 1/4" to play with, you might not need to worry about the groove.
Other than as a weak spot in the axle shaft that is.%);D
Paul