Properly locking on a floor mount rack with a U-lock I have read recommendations to lock both the bike's frame and wheel. That's quite easy to do on larger racks, such as Sheffield stands.

The place I work in, however, has a floor mount rack to store bicycles, with semicircular shaped supports. If I park the bike with the rear wheel on the support, the derailleur gets a bit bent, which I believe may damage it (am I wrong?). If I park it with the front wheel, my U-lock will only manage to lock the wheel, which is not a good choice since it's quick release.

Under these circumstances, is there a technique to securely lock it without damaging the bike?

The rack I mention is similar to the one below, but it has no bolts.
In a situation like this, I usually don't fit any of the wheels in the support; instead I park the bike over the rack and just to the left or right of a loop, so that the lowest part of the frame is in close proximity with the loop. I then lock the frame and a wheel with a single cable (a U-lock might not work). Alternatively, you can place the rear wheel next to the loop (with the loop on the left side so it doesn't clash with the derailleur), and pass the U-lock through the wheel, straddling the chainstay and thus locking the frame as well.

In short, just forget about the fact that the rack is designed to accommodate a wheel in a certain way. Use it as a piece of metal you can lock your bike to.

Keep in mind though, that by parking your bike next to a loop, instead of in it, you're taking a bit more space and making it a bit more difficult for others to park. Something to bear in mind if the rack is often full.

Also, as others pointed out, that doesn't look like a very secure rack. That doesn't necessarily mean you shouldn't use it; you choose how well you want to lock your bike depending mostly on how common bike theft is in your area, how attractive a target your bike is (especially in comparison to other bikes similarly locked), and how much risk of having to replace it are you willing to accept. I lock my bike on a rack just like this every night, but it's on a communal yard, not right on the street, and the bike is cheap.