even though I don't have a MB Pro2, as long as the MB Pro2 has a USB port, you should be able to mount this USB device into the MB file system.
And you should be able to modify (or create from scratch) the backup script to write to the external USB stick.
This may not be the "default" method for the MB Pro2 (meaning not covered by the scripts it comes with) but should still be feasible.
At the end of the day openWRT is a Linux variant, and what is possible with a (meanwhile) old MB Pro or a MB on RaspberryPi should in principle also be possible with the MB Pro2.
When you are familiar with Linux and SSH, you can connect to your MB and find out as what your USB device is recognized (after being plugged in

):
ls /dev/sd* -l
should show you all "special devices" (sd) what they are called under Linux and external USB devices are all treated as sdxn
x= a, b, c - just a "numbering", n=nothing, 1, 2 ... - the number of the partition on the external device.
Usually with no other external device connected, your USB device would appear as sda or sda1
e.g. with
mkdir /tmp/mnt/data/USB
mount /dev/sda1 /tmp/mnt/data/USB
(the data on) your USB device should now appear in the exported network share under \\METEOBRIDGE\data\USB
(this is assuming that the MB Pro2 has the same setup of its file system as the NANO, MB Pro and MB on RPi have)
Make sure your external USB device is FAT32 formatted.