Have done something similar in the past but it took me absolutely ages. It was for a specific design piece for college so not really something saleable as the time involved alone would have cost too much. I used individually wired bouvardia heads in white...It did look very effective.

A couple of ideas though I have just thought about is making either wire letters and binding on small bunches of gyp, sea lavender or another "filler" with small grouped flowers...

OR...

Do the dutch thing (cos they love their cardboard templates!) and cut the letters out of card (easiest to cut from an old anthurium box) and then cover them in either coloured webbing, sisal or even spray tac the letters and roll them in sea lavender/limonium flowers stripped from the stem which drys very well without losing definition...

Not sure how it will look though, but let me know if you try it