After much frustration and pain, I’ve found a way to convert a static image and an audio file into a high-quality YouTube video with the help of ffmpeg:
ffmpeg -loop_input -i image.png -i audio.mp3 -y -b 628k -acodec libmp3lame -ab 320k -vcodec libx264 -s hd720 -shortest output.mp4
Now you can upload mp3s to YouTube from any major operating system without worrying about poor transcoding. Just replace image.png and audio.mp3 with the appropriate files. Thanks to Kuukunen of irc.freenode.net/#ffmpeg for helping me work out an issue with uploading. 