Hi kubus_pl
Good idea and I'm sure you can come up with something which resembles the preview you've posted.
You could have the device icons as words as have it, and to stop Chameleon overlaying the device names you could use a blank font_small file, although unless you only have a couple of devices it will then be hard to identify which drive is which.
You can play with using devices_layout as vertical in the theme.plist to achieve vertically aligned devices although as they are quite tall (WIN NTFS for example) it might be worth considering having them listed horizontally across the screen to make it easier to fit on the screen..
For the boot menu, you can use an opaque menu background and place it wherever you want and it will be visible when activated. As for x/y coordinates etc. that's for you to play with when designing the theme
Good luck and I look forward to seeing a finished example sometime in the future.