Thanks to all, I have a simple mod that allows me to control the backlight (on/off only, no fancy fading) using a spare digital output pin.
The transistor is necessary because the backlight draws close to 100mA. The digital outputs can source (or sink) a maximum of 40mA. There could be tears..
Remove the trimpot, solder 1 x PNP transistor and 1 x resistor (nominally 1K)
In the attached circuit diag and code example, I've used digital pin 8 to control the backlight.
Software still isn't entirely resolved, though I have a simple workaround.
I'd like to use guix's UTFT library mod, but it doesn't seem to toggle pin 8.
I don't yet know enough about writing to these port registers to change it nor can I find good docs about port/pin mappings.
( I WILL fix this because it's a better way to do this).
For the moment though, I left guix's library mods in place (they seem to shut down the display, though not the backlight) and added a line in my arduino sketch immediately after the call to turn the LCD on or off.
const int backlight_pin = 8;
const int backlight_on = 0;
const int backlight_off = 255;