Another thought occurs at this point, though: maybe this could be solved from the steel side. Rather than creating new object types or having potentially misleading graphics, why not just make it so that, in the Editor, steel blocks can be set to override whatever is overlapping them via a simple checkbox that can be applied per-instance? The end result will be essentially the same anyway.
Because this raises
exactly the same problems as Simple Autosteel, except that it's now being set per-steel-piece instead of per-level so it's even
less consistent.
maybe terrain should never be allowed to overlap a steel block in NeoLemmix
Why? In the example you give - it may produce a somewhat awkward shape, but it's perfectly clear what's going on.
I can imagine it would be easy enough to hard-code steel to always "ride up" to the highest placement index, and thus always be drawn last.
And then two seconds later, you remember that eraser pieces exist and would not work nicely with this idea.
Ultimately - I don't see anything wrong with NL's determination of steel vs nonsteel, or NL's ability to overwrite steel, as it already stands. This is purely a problem of some people (I say "some" because WillLem is not the first, and probably won't be the last, to raise this issue) being attached to the way L1 does things. If it weren't for L1 having a few levels where they used not-intended-as-steel pieces to decorate intended-as-steel ones,
we would not be having this discussion - much like we would not have the obligatory once-every-few-months discussion about timed bombers if L1 had also had them as instant. While it can be advisory at times, I do not consider what L1 does to be
binding on NL in any way, and as I have stated - I believe there is an objectively correct answer for the right way to handle this situation in NL. If regular terrain overlaps steel, it should be because the regular terrain is
intended to cancel out the steel. Otherwise, remove it or put the steel in front (and before you say "but why should I have to put the steel in front?", let me remind you that you just suggested
forcing steel to be at the front always).
With all this being said - it is your pack, and up to you what you do with it (and being a conversion of an official game does not change this, it's still just another community pack as far as NL is concerned - only Redux has special status there, with the Intro Pack likely to join it when 12.11 stable is released).
The other thing I'd point out is - to "avoid affecting gameplay", the moss needs to be further from the edges than you may think. Having it less than ~16px or so from the left or right edge can affect basher terrain checks. And if there's any of it 6px or less from the bottom, it is possible to create a setup (provided enough other skills exist, of course) where the lemmings step up into it.
And I'll stress - unless you're specifically making a level that takes advantage of these, the physics results explained here are generally going to be harmful, not helpful, to your levels. Given that NL's philosophy is "physics come first" (and then, anything that physics hasn't dictated either way, can be used to create nice aesthetics, with NL
generally supporting such as long as it doesn't interfere with or create confusion around the physics), this leads to a conclusion of "don't do it, is the safest option".