MapNotes provides a plugin feature that makes this easy for me to do.
MetaMap does not appear to have that option.
You could certainly overwrite the icon files in MetaMap with those I provide and see what happens. I don't want to release a MetaMap hack though, or I'll end up supporting broken and borked installs.
I am releasing a new Beta of MapNotes (Fan's Update)
v4.10.20200.Beta.01
In this version, I am trying to allow people to select their own colour choices for MapNote Text.
i.e.
can now choose your own Text colours by Right clicking the colour selection squares on the Note Edit Frame
<Alt>-Right click the selection square to reset it to its original value
In order to do this I no longer use 10 different colour texture images; Instead I just use 1 white image and colour it in based on the MapNotes_Colors data.
Hopefully this Beta should still be compatible with your MapNotesIcons plugin, but I thought I would warn you.
Obviously your AddOn should still work to change the Icons.
As far as the Colours are concerned, then your AddOn can still be used to change the DEFAULT colours, but their is no need for you to actually change the texture images for the coloured squares.
In other words, the folllowing three lines of your AddOn are now unnecessary :