I happen to have a 9volt pre-amp tube 6CG7 lying around in my stockpile that I got at an estate sale a few years ago. The pinouts are the same as the 12AX7A used in the Safety Valve. The heater voltage is only 6.3, which is easy enough to take care of with a voltage divider. My bigger question/concern is the capacitance is quite a bit higher. It’s a “medium-mu” vs the “high-mu” of the 12AX7A. It’s also meant for a much different purpose, controlling oscillators in TV receivers. I think it might be worth experimenting with. Does anyone with more experience see anything else I might need to adjust in the safety valve circuit to not totally blow things up? Or what parts could be switched have the biggest impact on the results I might get out of it?
Big no, a voltage divider is unable to supply current. You will need to add a buffer or a voltage regulator.
I do it all the time though. You don’t connect the heater to the voltage divider, but rather make the heater one half of the divider 
It works just fine like this.
The 6cg7 heater wants 6.3 volts at a whopping 0.6 amps.
If you use a 12v power supply, put a 10ohm resistor in series with the filament. this resistor will constantly dissipate about 3watts, so choose a beefy one.
Make sure your power supply is up to the task and can handle the inrush current.
Any dual triode should work. I tested ecc81 (12at7) ecc81 (12au7), ecc83 (12ax7) and ecc85 (no american equivalent, mu about 60)
12au7 works best at low voltage, least pronounced gating effect, but also lowest distortion.
Also, the 12xx7 tubes will only draw 150mA at 12 volts, they WILL be easier on your power supply
Great, thanks for all the good info on this. I am going to try bread boarding this with my bench supply feeding the heater first. I have a 5V rail in my power supply, so I might try that too. Though I am guessing it will be quite a big draw on that rail and probably wouldn’t be able to keep up. I guess I am going to need one of those big ceramic resistors before I really start digging into building this.