Hello fellow Arch users, and those affected by this library issue.
I have a crude but workable solution to this problem, which allows for using an older pango library without obstructing the rest of the system. It works for me, YMMV, etc.
Thanks to @EquateBrand for determining what pango lib was necessary.
I did the following:
cd ~
mkdir .mylib
cd .mylib
wget https://archive.archlinux.org/packages/p/pango/pango-1%3A1.44.1-1-x86_64.pkg.tar.xz
xz -d pango-1\:1.44.1-1-x86_64.pkg.tar.xz
# tar doesn't like the colon, and I was lazy:
mv pango-1\:1.44.1-1-x86_64.pkg.tar pango-1--1.44.1-1-x86_64.pkg.tar
tar xpvf pango-1--1.44.1-1-x86_64.pkg.tar
LD_LIBRARY_PATH=/home/hesperaux/.mylib/usr/lib/:$LD_LIBRARY_PATH ~/Downloads/plexamp-1.1.0-x86_64.AppImage
Obviously you can change where you extract stuff, and fix the paths how you like. But this will override the system library path for pango. I did a quick test and PlexAmp does run fine.
When I don’t set the LD_LIBRARY_PATH, this still happens:
╚═══ [03:36:46 PM] $ ~/Downloads/plexamp-1.1.0-x86_64.AppImage
installed: X-AppImage-BuildId=1eb65ac0-f46d-11a8-2a14-3f03bf545e72 image: X-AppImage-BuildId=1eb65ac0-f46d-11a8-2a14-3f03bf545e72
(plexamp:104862): Pango-ERROR **: 15:36:50.217: Harfbuzz version too old (1.4.2)
and:
╚═══ [03:37:28 PM] $ pacman -Qs pango
local/pango 1:1.44.7-1
…which tells me it’s doing its job.
This is a crude workaround; I’d still like to see the real fix (no holidays, Plex Labs guys! 
). But do enjoy the holidays first. Happy holidays all.
