Your suggestion is great for the PRS
505, recycledelectron, but this thread is over a year old and refers to the PRS
500 which was then just released.
The PRS500 doesn't function as a USB drive (that's one of the big new features with the 505), so you have to have Sony's software, or libprs500 to manage internal memory on it.
If I recall correctly, the reason Bob was looking for this is that some of us (I'm one) would like to be able to remove books when we finish with them just for obsessive neatness' sake ... too bad that obsessive neatness doesn't extend to how I keep my office ....