Originally Posted by bpa
So how about putting your icon value a relative path using "../../.." (as many ../ as required ) to go up levels from Favorites icons before adding your path. Haven't tried this so may be totally wrong.
That was a really neat bit of lateral thinking. So, I tried using ../../../../../../../../mnt/myusb/images/myIcon.png but it didn't appear able to find those. After doing it, I did notice the following error in LMS' server.log, which didn't appear when using the original relative path

[20-11-09 18:42:34.1925] Slim::Web::Cometd::handler (421) errorNeedsClient: 00:04:20:27:da:b8, status, -, 10, menu:menu, useContextMenu:1, subscribe:600

I then hit upon a different workaround, which is admittedly a bit of a cludge. I added the following User Command in piCorePlayer's Tweaks page, to repopulate the volatile images folder from a folder on my mounted USB drive.

\cp /mnt/myusb/images/*.* /usr/local/slimserver/Slim/Plugin/Favorites/HTML/EN/html/images

This works. It's a bit ugly but, unless anyone's got any other bright ideas, it will achieve my end goal.