I would NOT recommend using body filler on any type of urethane bumer for any reason, because it WILL be only a matter of time before the "bondo" cracks out or completly falls out of the letters. Body filler is not really desinged to be put on urethanes, you can get away with it if it is only a very very thin layer but with the thickness you would need to fill the letters in it will crack out. You could deffinetly fill it in with a plastic welder if you have acsess to one and the right filler rods.
I would recomend you getting some plastic repair material from SEM. It is a two part urethane that is speccially designed for repairing plastics. Then just properly prep the bumper and fill it in with that, and then just sand smooth and prime and paint, and i garuntee it will never crack out.
well maybe you are talking about a different kind of plastic welding than i am famillar with. The plastic welder i use is like a sodering iron sortof with a hole through it that you feed the repair rod through after you heat up the substrate, and it "welds" the repair rod and substrate together, (or the two pieces of plastic together with the repair rod bewteen them for say a rip or tear in a bumber). And that would just be a lot of work, not hard but tedios, which is why i would just use the SEM repair material.
Please inform me if you have a different way of plastic welding 'cause i am genually interseted in always learning new techniques that work well, especially with plastic repair.