Hi Veggie,
You should be fine at 50Hz, just connect the 50Hz windings, and ADD CAPACITANCE on the DC side of the bridge diode as needed to boost the ouput voltage sufficiently. You can use motor run caps most safely. Even a small amount will boost voltage. If you go to higher levels of capacitance, the voltage boost tapers off, but you can then use electrolytic caps of 100V rating or so, since the peak voltages will be reduced. Peak voltages of my ST-3 harmonic with no capacitance were in the 400V p-p range on 1/2 hp motor starting surge. As you add capacitance, the peak will diminish substantially.
The IOTA has notoriously horrible power factor, chopping the upper part of the waveform off, but the stock ST generator harmonic regulation system copes with this well as long as you have enough head room on the capacity. You certainly should be OK for your plan.
Since most switching supplies will run on 120VDC quite nicely (my home is 120VDC), it might be possible to reduce your speed even further, by converting to filtered (inductor and caps) DC before the IOTA. If the harmonic was insufficient at say 40Hz, a modified version of my simple AVR design would keep the DC level up to whatever is needed. A bit of a project, but perhaps your 50Hz solution is sufficient.
Another way to allow allowed lower frequency would be to open up the IOTA and increase the high voltage capacitance after the bridge rectification.
Best Wishes,
Bruce