SuperSU root is working fine on Android L, it just needed a small change in script. Chainfire posted about it on twitter here. You can grab the patched SuperSU zip file from the link below and flash it via CWM/TWRP recovery.
Patched SuperSU zip download link: http://www.mediafire.com/download/sxtpct6p3of6zk3/SuperSU-v2.00_for_Android_L.zip
Instructions: http://www.theandroidsoul.com/root-android-l-developer-preview-nexus-5-nexus-7-2013-patched-supersu/
Credits:
@Chainfire for SuperSU
@tasssadar for the fix
@TheAndroidSoul for patched SuperSU zip file
Patched SuperSU zip download link: http://www.mediafire.com/download/sxtpct6p3of6zk3/SuperSU-v2.00_for_Android_L.zip
Instructions: http://www.theandroidsoul.com/root-android-l-developer-preview-nexus-5-nexus-7-2013-patched-supersu/
Credits:
@Chainfire for SuperSU
@tasssadar for the fix
@TheAndroidSoul for patched SuperSU zip file