It seems reasonable to me, but I'm not the one to sign off on something like this.  I've nosied someone who will have an opinion :)

Would doing:

    mybytes = bytes(random.randint(0, 255) for i in range(size))

be faster enough to be workable?  I suppose that might build a list under the covers...maybe preallocating a bytearray and filling it with ints would work better.