While it might seem slightly complicated using our calendar sync feature to TP (free) and Intervals could give you what you need including sync to your hammerhead. This would be the most painless process other than just using TrainingPeaks, or TP and TrainerDay.
I am reading on the intervals forum and their is no public API for hammerhead so it seems via TP is the best choice. So the only key issue with this approach is you must add all your workouts to our calendar. Once added you can actually move them around in Intervals and they will also move in TD and TP.