I have ffmpeg compiled with On2 VP8 support (libvpx-vp8). I was wondering how I can create my own rendering profile in Kdenlive that will support the codec and container?

yes. choose something similar and click the icon that is document with a plus sign. Set extension to webm. Remove any "f=" option. Change "vcodec=libvpx_vp8" and "acodec=vorbis". Ideally, also try adding "vpre=/usr/local/share/libvpx_vp8-720p.ffpreset" or similar - look to see what presets were installed.