persistent systemwide custom fontmap

[Karl Berry]

    is this an issue which has to be addressed
    to the package maintainer on arch?

Font map files are one of the things where every distro seems to do its
own thing. So yes, I think you should check out arch documentation/help
on this.

As for upstream TeX Live, we do not merge updmap files any more (for
some years now), so one can have an updmap.cfg in, for example,
  /usr/local/texlive/texmf-local/web2c/updmap.cfg
(or any other TEXMF tree) with one's local fonts. --good luck, karl.