I have removed a bathtub and I'm replacing with a shower. The shower tile installer asked that the new drain be placed in the middle of the floor. There is a heating duct that prevents placing the drain trap directly under the drain. Is there a problem if I run an 8" section of drain pipe to a location where I can install a trap and then continue to the main bathroom drain?

That should work. I am assuming that your shower is going to get a tile and grout job. If not, you can buy fiberglass shower pans made to fit an old tube opening. They are called retro fit pans and run about $280.