At risk of revealing my lack of "software prowess", I'm not actually sure how to do that properly / safely
But I also made a mistake in my above post, it should be:
register "usb2_ports" = "USB2_PORT_FLEX(OC1)" # FPR
register "usb2_ports" = "USB2_PORT_FLEX(OC1)" # SD
Port 4 appears to be the internal one. I had enabled all three and both the SD and the fingerprint worked.
I've now confirmed that both of the above are correct and work cross-platform.
@harryk I saw that you added these changes to your repo already, sorry for the mistake. Please update.