The loop offset is meant to be used when you want to override the default loop configuration in a sample. For instance, I needed once to use the Harpsichord samples to create a new instrument, because the sounding of this instrument is similar in many points, but with different effects and onset. So instead of duplicating the samples ant setting them individually, I just duplicated the harpsichord entry and set it to sound as I needed, changing the sample starting point and its looping point.
As for the negative values, as well as any value out of the sample bounds, the synthesizer just calculate the position of the loop by shifting the point to somewhere inside the sample bounds, e.g. imagine a sample of size 5000 (valid values are between 0 and 4999), then you set the loop starting point to -500 and ending point at 9800. The real loop will be between 4500 and 4800.
Hope that helps
EDIT: I'm not sure, but I think the OFFSET is a value discounted from or added to the default configuration of the sample, e.g. if the loop starts at 10 by default and you set in the instrument a loop start offset of -6, it should start the loop now at 4. But as I stated I could be wrong, so just to be sure, run some tests to make it clear.