Revisting all of the 6X5 discussions over on ARF, I think I'd go with diodes and just leave the rectifier heater glowing for looks. If my transformer were running hot, I'd leave the 6X5 heater disconnected. With diodes, as long as the B+ measured at tube plates is within the tube ratings as determined from a tube manual or a lookup site like
http://www.nj7p.org/Tube.php
you should be OK. If you need to drop voltage, just figure by Ohm's law. From tube data, add up the plate currents of all the tubes, figure how many volts you need to drop, and R = E/I. Then to figure the size of the resistor in watts, P = E x I. I would double this wattage rating to make the resistor run cooler.
As to tube socket terminals not shown on the schematic, sometimes a particular terminal is not used on a tube. That leaves the socket terminal free for the manufacturer to use as a tie point in the wiring.