I Heard You Want New Quake Injector
It uses Qt/C++, has BSD license, crossplatform (at least it compiles and runs on windows and osx). No binaries ATM, sorry.
Bugfixes and pull requests are welcome.
It can search&sort the database, show info&dependencies for map archives, download&extract them from the quaddicted, run maps listed in an archive (in quaddicted xml actually), show about dialog.
It needs dependency resolution, uninstall routines, an icon, and maybe installed/downloading tracking to be on par with quakeinjector.
Then it's up to Spirit&Co to add some nice stuff to quaddicted so the software could use that (like rating maps from the client or filtering maps you not rated yet, and quake://archive/mapname/ URLs).
ETA of feature complete beta is autumn/winter 2016, I need some time to get bored with the new doom.
If I can't pull this off by that date then "when it's done" is a pretty accurate release date.