The Honey Badger ran a 1mm thick Aluminium door finger plate along the front of the HDPE wedge and that did OK when the plate was wholly on the wedge as it was supported. At the Maidstone 2017, I moved it forwards and put it on hinges much to the same effect as you plan and it buckled horribly. I could straighten it out with a hammer but really, I'd advise against it. Steel might be better but I don't think it'll stand up at 2mm.
Interesting. I feel I should clarify though that with the steel piece, the top half of it is bolted on to the HDPE front as opposed to being hinged, sorry if the drawing didn't express that clearly. If you were able to use a 1mm piece of Aluminium scraping along the floor, then I assume that a 1mm piece of steel should hold up as well?

I'll also get onto the event organizers concerning whether a locking bar is necessary or not; I can probably have it go through the front claws where they meet the front of the lifter, though I don't know if something will break or not should I operate it (front claws could be ripped up, gears in linac could break, etc.) It seems from what I've been reading on other build threads that FW lifters seem to get away with having no locking bar, but the "clamper" part of the robot's design may pose more of a health and safety concern.