Many thanks Grok. In fact around midnight last night that's pretty
well what I came up with as my preferred working solution for now!
One small difference is that when I want one of the icons in my My
Places > Custom Icons folder, it seems I can:
1. Open its properties and close again (with OK, not Cancel).
2. Make the new placemark in the usual way (navigating to its location
and hitting Ctrl+Shift+P.
3. The custom icon from step 1 will be the default, so I just accept
that.
I have also used Macro Express Pro to write a macro (the closest I get
to programming!) that I can trigger from the Icon dialog. It browses
to a folder called \Favourite GE icons, awaiting a choice. Although
that's a bit faster than doing it entirely manually, on balance I
prefer the My Places method.
But neither are as good as adding icons permanently to the built-in
dialog display. So I'm still keen to hear from anyone who knows how to
do that please.