Any text can be verified as far as spelling with scancodekeypressed=x to detect each letter and then direct them to another state.
If the right letter is pressed then it takes them to a state that checks the next correct letter.
If the wrong letter is entered, then you can go to a state to take whatever action neccessary. (display hud, make sound, etc)
You can use activated states with the scancodekeypressed=0 to determine whether they have pressed an incorrect key rather than using a long list of scancodekeypressed checking for each incorrect number. If that statement confused you, then you're not alone. (it is tricky)
A separate script would be needed for each word used, and that is the real limitation here.
As Xplosys recommended, multiple choice is a much better (easier) method for text input in FPSC.