Seperate fuses could be costly in the event you lose the plate supply and the screens are still powered .
Aren't the screens powered by the same B+ that powers the plates? If not, what happens if you lose a rectifier?
IIRC, in the Valiant and the Viking 2, as well as many other BA rigs, the final plate transformer is only on when in transmit, while the low-voltage, bias, filament transformer is on whenever the rig is on. So the problem has already been addressed.
73 de Jim, N2EY