Can I straighten bent rims between two flat surfaces?
This is not going to work.

You won't permanently change the shape of the rim by just smooshing it between two concrete discs, as in order to permanently bend metal you need to exceed the yield stress of the metal and plastically deform it. This means that to bend something to a shape, it has to be precisely bent further than its ultimate designed bend, then elastically bend back to the destined spot. Your concrete discs will not achieve this.

It is tricky enough to form a suitably round rim from a perfectly straight piece of freshly extruded, homogeneous aluminum. Once the rim has been bent, it will have been cold worked, and the mechanical properties will not be uniform along the entire rims length. Thus, trying to precisely bend it back into shape will be impossible as the yield strength and elastic behaviors will not be fully known throughout the entirety of the rim.

Trying to heat treat it won't save you either as that will cause the rim to get softer as it will undo the designed-in cold working of when the rim is created at the factory. 

Additionally, this scheme, even if it was possible, would only adjust side-to-side trueness, and would do nothing to adjust vertical trueness. You'd also have to remove the hub and spokes, and at that point you might as well just get a fresh rim. In fact if your rim got bent while it was a wheel, many of your spokes are going to be wrecked anyway.

If a rim is significantly bent, it's only future destination is recycling.