Can you feed a solar controller from a charger AND solar?

"I don't see why not"

... likely because the answer wasn't written down on a faq's answer sheet.

But perhaps it could be done? One of these would maybe work?

400w 40v 10a power supply

400w 41-56v adjustable

Of course, it would be interesting to see what the mppt would do, with no maximum power point on the knee to track. Also, as pointed out, when in pwm mode, the high voltage would likely be passed thru to the batteries, without being bucked down... in that case you'd really be bucked. :)