Perhaps, instead of the 4N26, try a Til300 along with a voltage divider using high voltage resistors or a number of resistors spaced appropriately. The Til300 is a precision linear optocoupler that can be used to acheive a consistent transfer function. Here's the datasheet:
Oops, that part is obsolete -- in fact, the whole technology seems to be obsolete.
Perhaps digital isolation is the answer. Check out the following PDF and then consider some sort of voltage to pulse-width function on the HV side fed to the digital isolator. Measure the pulse width on the isolation side and convert that to a voltage reading.
