Surely a 5 wire unipolar is a Bipolar if you ignore the common centre tap. As such, this can be used with the drives you refer to. See diagram.
A 5 wire uni cannot be run in high-speed bipolar mode, but the 6 wire ones can.
I'm still learning this stuff, so correct me if the diagram is wrong, and if so, why can't the 5 wire unipolar be ran as a bipolar.
Thanks for your comments.