I did some more calculations, thinking and testing. Unfortunately, if I modify Levithan's scheme a little (for example, making the first blocker as well as the second blocker participate in steel failures), turns out it's possible to penetrate up to 24 pixels down from the surface. And turns out that is just barely enough to allow a basher bashing underneath to connect with the penetration. (The basher cannot start bashing any higher or the steel area will stop the bashing. Starting lower and the basher tunnel won't connect with the penetration.)
So even the 25-pixel steel plate is apparently not quite enough. On the other hand, I really don't want to enlarge the steel areas any further.
So instead I'm considering the possibility of hacking the game to remove the steel failure glitch. Since it's already necessary to hack the game for more mundane things like disabling the implicit level repeat mechanism (aka "oddtable"), there's no reason not to hack further. I doubt anyone here misses that glitch anyway, and we stipulated no glitches required for the remakes. This simply enforces one aspect of the rule.
I don't know yet how doable it is, but I'll definitely look into it.