[Mondial]

Mondial

if a spare winding could be added to a transformer to separately feed the filament of the 6X5, than that would not be a problem. You could even explicitly short them together.

Yes, that would certainly work, but it would add to product cost, and reducing cost was why the 6X5 rectifier was used in the first place. 

In fact, the 84/6Z4 and 6X5 were originally designed as rectifiers for use in car radio vibrator supplies, not in AC powered sets.

Manufacturers like Philco and Zenith soon found it was cheaper to use these tubes instead of an 80 or 5Y3 which required a separate filament winding in inexpensive low power sets. Why they continued to use the 84 and 6X5 in larger consoles is another story. It could only be to save money.

Putting 400VDC between the grounded filament and the cathode of a rectifier tube is really not a great idea. The insulation will eventually break down and short. Its just a matter of time.

The 5V4G and 5Z4 also had heater-cathode construction for low forward voltage drop, but they both purposely tied the cathode to the filament internally so one had to use a separate isolated filament winding.