The LTZ1000 reference circuit needs a divider of some 1:12 to 1:15 to set the oven temperature. This are usually 2 of the most critical resistors. It is mainly about long term drift, not so much the instant TC, but a good TC is also wanted.
The required stability is somewhere in the 5 ppm range resulting in some 0.1 ppm drift contribution for the final reference voltage.
There is no need for a really fine adjustment, though a little adjustment range may be nice. Also linearity is not important.
There are a few possibly gotchas. So It is probably keep the discussion separate here and not the the long LTZ1000 thread.
The general setup would likely be something like:
1) A buffer for the 7 V ref voltage. Likely a relatively large buffer cap at the output to filter charge injection from the switch.
2) A SPDT switch to choose between 0 and 7 V
3) A resistor of some 100K and low pass filter.
4) A buffer for the output. Likely an AZ OP, as the filter is high impedance and drift matters.
5) Some filtering (against AZ OP artifacts) and resistance to get an output impedance comparable to the usual 1K:13 K divider.
The PWM signal would likely come from a small µC.
In addition one would likely need means to turn off the heater for the initial time until the DAC voltage has stabilized after power on.
Overall the demand is not so extreme, so chances are good it would work. It is more the question how much effort is needed.
Chances are one would first test the PWM DAC without the reference and only than if everything works use it would a LTZ1000.
There are other uses for a simple PWM DAC, like getting a stable 5 V , 2.5 V or 1 V ref. output. The 7 V to 10 V step is a little different, as the PWM DAC would be in the feedback. Still some point may apply. There was a thread on a 7 to 10 V step with PWM DAC:
https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg2092108/#msg2092108One point open to discussion is whether it is worth compensating part of the ripple with an added inverted AC coupled signal. Not sure how much it helps.