I think cuffs would be too restrictive overnight and without padding they'd tear up the wrists. I think @Tobbe once mentioned locking mittens (in the context of female chastity of course), but they'd make a lot more sense here than cuffs. You can get medical ones that force a flat palm, they were apparently used in the prison system long ago for a very similar purpose.
However, @Renita is correct that they'd have to be unlocked each morning, but if the gf really does hate belt-fighting she might put up with the (minor) inconvenience. Maybe they'd be kept, like @HHelen's bra idea, in reserve to be used if and when they're needed?
I can certainly see how they'd be useful in the example you gave, but I don't know how @Verrouille and his partner handle cleaning periods.