ever-so descriptive thread title there.
most arches need to follow the texture quite closely (eg. DM-Pressure, DM-Curse][ etc) so the only thing to do is make the shape in the 3D editor & either save the shape there or save the resulting brush for future use.
there's going to be some sort of prefab feature in thenext UED AFAIK.
If you mean something like diagram A, then yup, that can be done as a brushbuilder, but to give it any sort of flexibility would take rather more time coding than to draw it in 2D & set the resolution of the curve.
Although (long-running discussion elsewhere) the curve won't be a true circle. (I'm not convinced that Bezier curves are capable of describing a true circle. Approximation, yes, but not a mathematically true circle. If anyone does find a proof, for goodness' sake,
write it down BEFORE going out to fight a duel, okay?)
I plan on making a brush for shape C, because it'll come in very handy for rounding internal courners.
as for B, I can't remember why I drew it.