I've thought of this exact same thing, because of the exact same episode.
I ended up interpreting "cast no shadow" as not being exposed to light under normal circumstances. In other words, a lvl 5 vegan can only eat vegetables that has the edible part under ground. Potatoes, carrots, etc. Fruits and berries are no-go. Maybe even there's a requirement that said vegetables have to be harveated and eaten at night time to avoid shadows.
Also, I think you intended to put the word "eat" at the end of the title.
I have both that album, and an album of the guy that song is about.
EDIT: for those who don't know, on Oasis - Morning Glory, there's a track named "Cast No Shadow". It was written as a tribute to Richard Ashcroft of The Verve, a close friend of both Liam and Noel Gallagher.
https://en.m.wikipedia.org/wiki/Cast_No_Shadow_(song)