Hi Dan
The Q's I listed are basic to any type decision about that wall. I don't t think you can move forward properly w/out answering them all, although most would likely be fairly obvious. As you probably know, any appreciable weight the arch carries will be transferred to lateral forces - ie. the arch will try to spread left and right and it's integrity depends on the existing wall being able to accommodate (resist) those lateral forces completely and with a margin of safety. Because of it's "low rise", that lateral force will be much larger than for an arch with higher rise. That is assuming the vertical loads down each side of the opening are fully and safely accommodated.
It's been there for a while (!) so depending on your list of changes to that structure (a list which you're _certain_ you won't add to in the future), then if those changes don't change _any_ of the structural elements of that wall, the opening
should remain as good as it ever was. You'll notice there are a few variables in the above sentence, but not beyond your ability to verify in situ, I think. If the wall extends up two stories as one single structure, incompressible, then the part above the opening, if it's tight and in good shape, will carry a lot of it's own weight and bridge the opening simply because it will refuse to deform in the way necessary for it to fall into the opening. If the wall is "broken" at the ceiling, or if the structure above the opening is not rigid incompressible brick, then the arch may end up carrying most of what's above it - because in that case what's above it will tend to "scrunch", deform, down into the opening with it's own weight if nothing holds it up.
From your point of view, identifying the "structural elements" of the 1st floor wall, the concepts and practices, may be the biggest question. . Whether you feel comfortable and/or have time to go that course yourself, depends... <g> If you identify the structural elements and verify them, your decision is pretty much made, viz the arch. Whether any wall above will bridge the opening safely just by itself, based on the (2nd floor) wall's own integral rigidity, is a judgement call which isn't as easily cut and paste. That, if it becomes an important question, looks to be something that that almost certainly needs an experienced eye and mind, on site in person.
Regards,
Rufus