Hm... This sounds like a neat idea, but browsing around a bit - I think there'll be a whole bunch of complications to implement it in that manner. The current implementation is very simple - it just draws text and sets the colour (rgb + transparency) for the drawn text.
I can't find anything that would allow me to set tint per ghost entity, or to get (programatically) some kind of mask/sprite that I draw on top of the entity (so it would overlap exactly with the underlying ghost).
However... It could be possible to implement some kind of nicer looking sprite. It might affect performance slightly (I assume drawing text is cheaper than drawing sprites). But that could be somewhat offset by having support for either variant.
That being said, this is probably not something I will be looking at anytime soon, but... If you are able to come up with some kind of prototype with screenshots (particularly in case of sprite alternative) on how it would look like, that could be a starting point.
In case you want to experiment with some basics around just changing how the text is rendered (or what character is used for drawing), you could have a look at the approvalBadges.lua
file. Just don't use it against your main save game. :)
Best regards,
Branko