As you describe it, it seems that you only can use a custom text routine. Let's say that S is the offset to the starting letter, E the offset to the ending letter and A the address of your appvar ::While S<E
:Disp {A+S}>Char
:S++
:EndOf course, it can be optimized.

EDIT : I don't remember if this automatically goes to the next line if the right of the screen is reach ...

Can you put a null terminator at the end of every word/string unit (like was just recently brought up here and here)? If each small piece ended in a null byte, then you could display each one just like a normal string, given the pointer to its start.

If speed is not of huge importance, you would also then have the option of using stdDev() to get the pointer to individual strings given their index in the "list" of strings. Not sure if that would make it easier or harder for you, but you would have the option.