Is it OK to run my Princeton Reverb with the tank unplugged.
Yes.
I've been experimenting with different Reverb Tanks I have around the shop . . .
Only a Type 4A, 8A, or 9A will work with this amp.
Is it possible to use the Reverb IN and Out as an Effects Send and Return.
You would have a parallel loop instead of the more common series loop, but the wet signal could greatly overpower the dry signal so that it would sound more like a series loop.
The Send is designed to deliver a restricted bandwidth signal into an 8 ohm load. You would be giving it a load of maybe 100,000 ohms to 1,000,000 ohms. This load gets reflected back to the plates by a ratio of about 3,000 : 1. With a load like that, a very small input signal to the driver will overdrive it. A 12AU7 for the driver would help if you keep the Volume control on the amp very low. If you make a breakout box to load the transformer, you still have the restricted bandwidth problem.
The Return is designed for an input signal of 5mV or so. An effect signal is frequently 500mV or so and can go much higher. You would have to turn the Reverb control way down to prevent overdriving the Mixer stage and achieving the full rated output of the amplifier. Again, the bandwidth is restricted.
Other than that, it will work.