It should be noted that the open source version of osu! is still under heavy development and is not the currently officially supported client you can download on the osu! website.
I also fail to find any reference to what the version requirement for the OpenGL renderer would be (either client). The new one is using shaders though and I don't see any ES code around.
Performance would also be questionable. opsu! was worth trying, but barely gets above 20 fps on the Pandora. That was Java. Not sure if Mono would perform much better.
The current stable client runs above 60 fps on a Z3735F Windows tablet, but a whole lot more. In case that's useful as a reference.
Maybe in the future, though.
For fast code, McOsu would be preferable (C++). But I noped out of suggesting that client since it requires OpenGL 3.
Interesting, thank you for the infos! As you seems knowledgeable... how is the compatibility of these different clients, are they "drop-in" substitutes (compatible fileformats, interfaces etc)? Also, what makes osu! (or the other client variants) special / successful over the other earlier rhythm freeware /FOSS games (Stepmania, fretsonfire etc)? I'm still surprised by the size this freeware game's community achieved... can't currently really grasp what is the special thing here.