It has been a week with the references running at the heater temp continuously. Here's the data (taken after the DVM ran for 1 hr at room temp):

2019-05-28 168hrs at temp

Meter params: HighZ, 100PLC, AZ on (Null for 1V and 100mV ranges)

Set Point: = 0.62785

Value            Range
100.0068      100mV
1.000029      1V
2.49991         10V
4.99923        10V
10.00006       10V

So far so good. The 100mV reference drifted up by 1.1µV (0.0011%), tracking the upward movement of the 2.5V reference. The 5V and 1V references drifted up by 0.002%. The 10V reference did not move.

The box will remain powered for another 5 weeks or so.