You are only allowed to do this work and create a release of such an installer if you build and test it on all supported platforms with complete feature parity.
I kid! That’s not serious at all. You really don’t need to ask permission to do anything around here. Just do it if you want to do it. It’d start as a speculative experiment, which is pretty much how juliaup started. Build the thing, get folks to try it, make it robust, and build enough confidence and support for it to be marked official. Even that final step, though, is simply a proposed change to the website: Recommend juliaup by IanButterworth · Pull Request #1969 · JuliaLang/www.julialang.org · GitHub.
You’ll likely find the most success at building a community around the thing if you build it where the community is or work within the context of existing efforts.