Hmmmmm.... it's a great idea and I've seen the DS version do this. Problem is that the way Unity handles it's UI elements is REALLY HORRIBLE. This means that to keep the frame rate up I draw all the numbers as 2 elements (top and side). Also - fonts seem to be single colour so if I change the colour/shade of the font - it will change for everything.
What I might be able to do is remove the numbers for the lines that have been complete. I'll take a look and see what I can do.