I wonder if something like caulking would work. It is flexible. just fill the hole and sand down even with the rest. Although IDK how well that would work with the bumper being so flexible and the paint sitting on top. Could fiber glass over but it woul have to be a really good repair so it would be noticed and then, cracking again.
I worked for residential/commercial painting companys for 4 years on and off and you know the caulking idea sounds like it might just work, hard part would be getting it to be perfectly flush with the bumper tho. Might take 2-3 application and drying proccesses. Although if the bumper does flex alot from summer/winter temps the caulking could crack like everything else mentioned
What about heating up some plastic (melt something rubber or plastic, pour into each letter?) That could turn out pretty cool if worked right.