No can do. Player chattext is the same text, font, and size as specified in options menu which is the same style used for Nicknames.
using showimg to display text is more for visual signs, not chat text, altough it could be done it is not necessary
The hardest part would be keeping it centered above the player's head
actually it would be nearly impossible since u cant get the size of the players screen + some other stuff which it needs to display the text like graal does.