title_body: Is there a specific term for a constructed writing system purportedly used for a conlang but actually for a natlang?
upvoted_answer: I don't think there's already a word for this exact subset of scripts.
  
  However, I think such a term would be a useful one -- we certainly have to refer to these sorts of scripts in the conlanging community -- and I think there are a number of options here.
  
  1. Circumlocution
  
  We could refer to these sorts of scripts in paraphrastic ways, like how you have in your question. "Transliteration alphabet" and "substitution cipher" both work for this purpose. However, I don't think this is a particularly attractive option, as these are long and rather unwieldy. Shortenings of these phrases end up being pretty ambiguous, as well. "Transliteration" in particular is usually used to describe completely separate things entirely, so using it in isolation to refer to these scripts would be confusing at best, and "substitution" is too vague.
  
  2. Neologism
  
  Since these sorts of scripts are really the neographic equivalent of relexes, we could also coin a new term like "relex" to refer to these scripts. Just spitballing, here are a few examples of the sorts of coinages that could work:
  
  
  transbet (from "transliteration alphabet")
  subscript (from "substitution script" -- though this word does obviously already have another meaning so that might not be desirable)
  rescript (from analogy with "relex")
  keychain script (because these sorts of scripts are easily used to sell keychains with people's names on them 'in another language')
  
  
  There are any number of ways that such a term could be derived beyond these, but I feel that in order to have a term that describes this subset of scripts, some sort of new coinage is necessary.