Presumably, if proprietary code isn't linked with GPL code, it's not a derived work, but two separate works under different licenses. If your situation can tolerate it, you might could write a gplserver running in a separate address space. Or, if it's hardware to hide, video drivers frequently provide a razor-thin kernel driver that does little more than let you mmap the device's I/O space, with all the action sequestered in a proprietary userspace library.