. Of course, there would be need for external temp compensated voltage reference,
external ref, yes.
my idea would be the sampler, integrator with cap, comparator , matched current sources and nulling system in the semicustom chip. The control logic would be outside in a cpld or fpga.
The converter would be a dual slope architecture so no mucking about with thousands of opamps.
A unity gain buffer (chopper stabilized). An analog switch and cap. Another buffer , an integrator with cap , some more analog switches and two current sources that derive from the external reference.
The key is the integrator and the comparator.
The rest is logic.
Short integrator cap. Close sample switch.
Connect known source to integrator. Release short. Wait preset time.(external logic)
Close to preset time (t-1) : Open sample switch
At preset time: switch from known source to the sampler output and time (external logic)
When comparator toggles (zero detector): end run.
Apply compensation math and conversion is done.
This is how all these high precision machines like a 34401 and 3458 work.
Might be worth a try.