It sounds as though the data transfer only needs to be mono-directional, not bi-directional. This alters the work involved in any of the interfaces, hence my comment. There is also the question of the sort of 'rate' required?. There were some posts, quite a while ago, on one of the 'embedded control' groups, about getting the FTDI chips working with qnx, and they did seem to manage it in the end. A Google search, might still find this thread. It still would be the easiest solution.
Best Wishes