Sorry for resurrecting an old thread, but I modded an HP HSTNS-PL14 (460W variant) today, and have a few additional details that might help someone else out.
So, I traced a few other resistors, and it looks like there are three resistors on the high side of the voltage divider feeding the Vref. One is 4.42k, another is 10 ohm, and another is 39.2 ohm, for a total high side resistance of about 4.4692k. On the low side, there is the 1k resistor, the 100 ohm resistor, and a 100 ohm potentiometer, for a total low side resistance of 1.1 k up to 1.2k. Back calculating gives a plausible 2.5Vref as output of the voltage divider (I could have just measured this in hindsight).
So, knowing the Vref, total high side resistance, and low side resistance we can use a calculator to adjust the resistors/potiometer to fit our needs.
By default the 1.1k - 1.2k low side resistance gives a (calculated) adjustment range of ~ 11.81V to 12.66V.
By bridging the 100 ohm fixed resistor, we change it to 1k - 1.1k for a (calculated) adjustment range of ~12.66V to 13.67V.
I went a slightly different route, and replaced the 100 ohm fixed resistor with a 16 ohm, and the 100 ohm potentiometer with a 1k. This gives me 1.016k to 2.016k for a wider (calculated) adjustment range of about 8.04V to 13.5V. I added the 16 ohm to lower my upper bound to about 13.5V since it seems like anything greater might require other mods for stable operation at > 10A.
In practice I have also found that anything under about 8.6V isn't very stable, and the power supply shuts down, probably due to an undervoltage condition.
After making these changes, I used my load tester and verified that my power supply is stable at 9V up to at least 17A, and at 13.5V up to at least 11A (that is about the limit I can test with my 150W load tester).