There ought to be something to beat the cracking problem, even if it's not LineX. I had the engine cover and a few other engine bay trim pieces painted on my previous car and the body shop primed, painted, clearcoated, and then sealed the plastic with some 3M high-temp resistant coating. The body shop normally warranties their work for a year, but in this case they were so sure of the product that they actually guaranteed me the paint would never fade or crack.

I'll have to do some research on what they used...