Would there be a way of making the C64 Mini use OTG

It's not possible with the standard C64Mini I think. The port on the back is configured as 'OTG' which if I understand correctly means it can be switched between host mode (which is what you want) and client mode (for use to connect to a PC) but that is only halfway configured. It doesn't work as client port if you power it via a USB port on your PC (no new USB device shows up). But it also doesn't work as host port, e.g if you use an USB OTG hub/cable you can power it but it doesn't detect e.g. an USB drive.

The behaviour of the USB port is determined by the boot configuration (script.bin) and the Linux kernel.

Script.bin is in nand, in a part that isn't managed by the nand driver (it's not nanda nor nannb) and is read by the bootloader and used to configure the system. This is setup to have the port as 'OTG' port:

[usbc0]
usb_used = 1
usb_port_type = 2 ---> Type 2 = 'OTG'
usb_detect_type = 1
usb_id_gpio = port:PH04<0><1><default><default>
usb_det_vbus_gpio = "axp_ctrl"
usb_drv_vbus_gpio = port:PB09<1><0><default><0>
usb_restrict_gpio = port:PH00<1><0><default><0>
usb_host_init_state = 0
usb_restric_flag = 0
usb_restric_voltage = 3550000
usb_restric_capacity = 5


The kernel also needs to be correctly configured to use the port as  'OTG' to get it working. For more info see: Section 'sunxi-3.4' at linux-sunxi.org/USB_Gadget/Ethernet. It shows that for 'OTG' support the kernel should be built with at least:

CONFIG_USB_OTG_UTILS
CONFIG_NOP_USB_XCEIV
CONFIG_USB_SW_SUNXI_USB
CONFIG_USB_SW_SUNXI_USB_MANAGER
CONFIG_USB_SW_SUNXI_USB0_OTG

However not all of these are set in the kernel configuration:

# zcat /proc/config.gz | grep CONFIG_USB_OTG_UTILS
# zcat /proc/config.gz | grep CONFIG_NOP_USB_XCEIV
# CONFIG_NOP_USB_XCEIV is not set
# zcat /proc/config.gz | grep CONFIG_USB_SW_SUN7I_USB
CONFIG_USB_SW_SUN7I_USB=y
CONFIG_USB_SW_SUN7I_USB_MANAGER=y
# CONFIG_USB_SW_SUN7I_USB0_NULL is not set
# CONFIG_USB_SW_SUN7I_USB0_DEVICE_ONLY is not set
# CONFIG_USB_SW_SUN7I_USB0_HOST_ONLY is not set
CONFIG_USB_SW_SUN7I_USB0_OTG=y
CONFIG_USB_SW_SUN7I_USB_DEBUG=y
#

So I think there's something missing :-(