I had glance your design. very well done.
You want alphabetic figure but
7-segment is not enough for it, I think.
maybe Hex display will doable.
Did you consider dynamic display?
If you have PSoC Designer, then look at LED-display module data sheet and
These coding of user module.
It might be helpful.
Another approach is make it by Verilog module.
That is very cool, I think.
I have not looked at your code, but basically an array with 8 elements/alpha character
with address mathing ASCII. Take a look at these as examples -
http://blog.microcontroller-kits.com/2012/11/26-c-program-for-7-segment-display.html This one uses a case statement,
I prefer an array, probably saves code space as well.