@Piers Host/OTG isn’t fully supported on the built-in USB due to complexity of the power (additional ICs, etc.) that would have been introduced to accommodate it.
However, Host support is possible with the external USB pins, though the current breakout boards only use it as a device. Software support for USB Host still has to be added as well.
There are plans for making additional breakout boards in the future. Some of which would include USB Host hardware.
Hope that answers your questions!