The final end user in the chain is going to do what they want, regardless of whether the OS is obsolete or not. I will make it a point of making sure they know there's less than a year before the OS is fully abandoned.
In the meantime, I am tasked with making sure that my code runs on what they have. I started down a road of trying to access the USB devices without libusb and by going directly to the device through the /dev/hiddevN path. The udev rules are able to set the access on the device node it creates. Of course, once you open the first can of worms you find several more inside - hardly worth all the effort. i gave up on that and looked at the hotplug mechanism instead.
The fix turned out to be pretty easy. I'm posting it here to complete the thread (I like seeing threads that include the final fix).
That version of hotplug used the 'usb.usermap' file to connect specific devices by VID and PID to a script. I added a number of device-specific lines to that file, each of which refers to the 'mydevice' script, and then wrote a quick 'mydevice' script to put in the /etc/hotplug/usb directory.
It works just swell. That was a much easier fix than the convoluted workaround I started trying and yes, it's all a throwaway when the user finally upgrades to a modern version of Linux. But users are always right, aren't they??