The idea is great.
I like it heaps.
From what I understand though you are suggesting a custom character on the fly? So what you want (tell me if I'm wrong) is for the LCDinfo core to search through the screen files, find any referance to that particular Ascii code, and add a custom made graphic in the same position?
It would probably be easier I think to implement this by using some LCD's ability to have custom characters loaded into them, and then just using those.
Anyway, just my thought on the idea... I might have misread something or just got totally confused somewhere, so feel free to let me know if I have.