Tested Shibby and AdvancedTomato version 135:
L2tp VPN [simple] *pass-through* doesn't work. Need to run occasionally a VPN tunnel from a laptop to my corporate VPN.
Same issue with a recent DD-WRT build.
However L2TP pass-through works correctly with in Asuswrt and Asuswrt-Merlin out of the box with default settings.