I finally got to test the video synth with a VGA signal generator, following this tutorial, and the results were far more pleasing..

I also realized that I had reasonable affordance with the voltage, and voltage fluctuations lead to interesting visuals :

Voltages between 5.5 and 12 seemed to work. I had initially been under the impression that I would need at least 7V, and was gearing up to use a dc-dc booster, but it seemed that perhaps directly using the fluctuating voltage from the solar panel could be a good option..

I tested the current draw and was pleasantly surprised to see how low it was :

37.8mA @ 8.3V
26.9mA @ 5.7V
with the battery charger i got 5.56V, sunlight wasn't shining directly into the window but reflected off the windows opposite

I forgot to take a picture without the battery charger, but it was about the same voltage (5.5V), with more fluctuation.. but for some reason the circuit wouldn't power with it. I need to investigate this more, on a sunny day..

I also played around with the dc-dc booster (though I probably won't end up needing it)

turns out alligator clips really don't make the connection well for this, it pretty much killed the voltage
usb worked well for boosting voltage, and I also tried holding pins to the input which also worked 

Next steps would be to try directly powering with the solar panel again, triple-checking everything, and in more direct/stronger sunlight.. If that still fails I could use the dc-dc booster, though I hope it mirrors the fluctuations with the input. (otherwise it has a tiny screw for adjusting how much it should boost to)