It might be easier compared to resolution-based downscaling, but it's not something I want to do. Partly because I'm wary of promoting something from "magically works" status to "solution" status until I fully understand why and how it works, but also because I think that the application of mipmapping is ill-defined for our (GUI) purposes.
Basically we want accurate, consistent depiction without discontinuities. Due to having reasonably-sized triangles right next to heavily compressed or stretched triangles, by using mipmapping we end up with vastly different texture mipmap levels being rendered right next to each other. This is bound to create discontinuities. Anisotropic filtering can improve things but apparently it works off of the same premise if it shows artifacts at all.
Without mipmapping at least all the triangles are textured with the same texture rather than different-sized versions of it. Additionally downscaling the textures might reduce the aliasing we're seeing, kind of like poor man's mipmapping that applies one mipmap level to the entire screen.
(And yeah, it seems that the accented borders serve as an important visual cue that is suddenly missing when going back to the default GUI. On IRC, I believe Cheese and I agreed that this is the sort of change that the default GUI should definitely have as well, even without entailing any other changes.)