I don't think designing a writing system is prescriptivist -- or at least if it's any good then it isn't.

Here's a good and recent example, a new, unified, orthography for Low Saxon. The way they did it is to take Old Saxon, re-trace the sound changes in modern dialects phoneme by phoneme and then assign glyphs to everything, which may be realised differently depending on dialect, say "sk" can be pronounced (English orthography) sk, shk, or sh. Mergers etc. are preserved in the over-regional orthography, though there's also a set of regular changes you can make to that universal orthography to get at dialect orthographies.

That is, what it's doing is simply to try its best to do both the history and the present of the language justice, to preserve nuance while providing regularity. Situations such as "loose" vs "lose" are perfectly fine because you derive the spelling of the adjectives from their roots (at least I assume why it's that way in English). Your dialect may, or may not, merge them in that situation the orthography doesn't say that you should or shouldn't do that -- only that you should still distinguish it in writing because there's people who don't merge stuff like that. People don't start to write "cot" and "caught" the same all of a sudden, either, they probably don't even realise they're pronouncing them the same.

Contrast that with the previous "standard" orthography (for the dialects within Germany, that is), Sass. It basically says "write it like you would write German" and that's right-out disastrous as German orthography just can't deal with the phonetics, merging and shifting especially vowels left and right away from what anyone is speaking. As such it was prescriptive by negligence, confusing many a learner.