Interesting question. I'll give you my short answer: I don't know.
More in depth, yes, pin 2 is the input to the inverter. I would think you'd want an isolation resistor between your proposed power amp in and its connection to the tube. I don't know whether or not the 68K resistor on the input jack would be right or not, but it might be fine. I don't know if the voltage would be right or not. The schematic I'm looking at doesn't show a DC blocking cap in line with pin 2. Probably a mistake in the schematic, but, if not, that could be a problem.
What do you mean by "bottom input jack"? My 200S has two input jacks, side by side, labeled 1 and 2.
All in all, it ought to be worth a try. If you know what you're doing, you shouldn't harm the amp or yourself. Of course, if you don't know what you're doing, you could kill yourself, so there's that.