I've thought of doing a similar solution with some aluminum bolted in there like that. I have mud flaps on mine now, but they start at the frame and hang down. They helped some through the years, but all the patching I did on my rear sill 18 years ago has rotted back through, even worse than before. Prior to that my dad had hung rubber mud flaps all the way down from the fender towards the ground, he claimed that may have helped the sill, and it was okay back 1996, but was beginning to rot through and bubble at that point. The problem with his long flaps was that if they did hang up on something, it had pulled on the fender some. So we built brackets and hung flaps on the frame.
The getting in there to wash it is a problem though. I thought with my aluminum idea I would leave a hole in such a way that little mud would get tossed back, but you could stick the hose, or pressure wand in it and still blast out the back. Either way it is a terrible mud catcher and hard to get very clean. I also think my accelerated rot came from preexisting hidden rot and the drastic increase in the use of magnesium chloride in my area for winter de-icing, and dust suppression on the local dirt roads, of which I have always commuted on each day. It makes the mud on them very sticky and caustic, it's really worse than the liquid mag on the highway, it allows the mud to actively absorb any moisture. When my dad was still daily driving my 40 back in the late 70's and early 80's mag was not an issue.
I need to do some serious rebuilding of my tub, again, in the future, I've though about totally enclosing that area and sealing it off, I don't know if moisture alone will rot it out like the bottoms of the rear barn doors and front doors at that point. However, it would be easier to wash out without that hidden shelf holding the mud.