I don't think he has ever been asked. When he left Hollywood in 1989, he did it of his own will. He wanted to raise his kids in Germany, but if he had stayed I guess he would have been a real contender for Zimmer for many years in the early 90s. These days, I have a feeling he would have been interested in hearing what Zimmer may have to say if Hans asked him to join his crew, but HF has stated that he thinks Zimmer is a workaholic with no free time, and HF would not like to live such a life. HF has turned 60 now, and I guess he's more of the relaxing and laidback type these days. But he has also said that he wants to work with Bruckheimer again. I guess JB has by now forgotten all about him though.
Travesty that most of his iconic scores still have no releases. Wonder why intrada or lala have not tackled them? Or maybe they tried but ran into red tape? After all these years you would think the studios would jump at the chance of reviving an old 80's blockbuster. I want them!