By using the statistics you can display an additional digit, but this is an averaged display of the buffered readings. You can also use the trend chart to extract data beyond 6.5 digits. GPIB and logging data also display beyond 6.5 digits. While the accuracy beyond 6.5 digits is not there, the precision can be useful in some cases. The biggest issue is keeping the 34465a thermally stable, within + or - 0.2 deg, when trying to measure beyond 6.5 digits. When measuring a 10 volt reference for example, my 34465a readings will increase by 4-6 uV per deg C rise. But I have found some limited use for extra resolution. Some say the 34470a while having a more stable long term reference, the internal tempco of the meter limits its usefulness as a 7.5 digit meter as well. Other than the reference, the 34465a and 34470a are the same and also share some of the same tempco limitations.
Edit, I found the ACAL is not really usable beyond 6.5 digits either. I found it best to use the ACAL when the internal temp is at or near the calibration temp and try to take critical measurements all at the same internal temp. That is, if the meter's internal temp is -2.5 deg C from last ACAL for the initial measurement, take all subsequent critical measurements at the same internal temp.