Can I connect a Solar Panel to augment my computer's power supply directly to a 12V line?
Most solar panels are set up to charge 12V batteries. So their optimum output voltage is somewhat higher than 12V, and their open circuit voltage is up around 20V. Below the optimum voltage, they are essentially a constant current source.

So if you hook the panel (with a blocking diode) to your 12V1, four bad things can happen:

One, you will be getting something like 20% less power than optimum. 

Two, you will be running 12V power (40A for 500W) down a long line from the panel to the power supply, heating up a lot of wire (the power loss isn't relevant, just part of that 20% above), or spending a lot on copper.

Three, if the load disconnects, you will (might. Maybe the PS has over-voltage clamp, but don't count on it) get 20Volts on your 12V1 line. 

Four, if the mains power fails (and the UPS gives out), the panels will be at best useless, at worst feeding under voltage power into the MPU with no 3p3 or 5V.

Trade these against a nice simple hookup, the panel feeding a constant current into the 12V line. Your PSU will just see less draw.

But if you run the panel the way its designed, charging a small battery setup with a 'grid tie inverter' near where the panel is installed, most of these issues go away. 

I (being a careless maveric with lots of cells but no money for inverters) would choose the direct connection, with a simple over-voltage shutoff, and solar panels chosen to deliver less than 400W. Probably custom built with just the right number of cells for max power at 12V.

You, being a bit-coin rich, responsible fellow with a view towards what to do with 500W when not running a GPU (maybe run an emergency wireless mesh node for when the power goes out gets my vote), would choose the grid-tie inverter approach.

As you are in Minnesota, consider the outfit up on the Iron Range that has got some press lately for their rugged solar panels.