To summarize the question, you want to get a analog signal across isolation. The bandwidth is 1 Hz, and you need about 1 part in 32k accuracy.
This needs to be done digitally. Analog methods good to 15 bits will be hard to find, and expensive if you do.
This low bandwidth is well within the range of delta-sigma A/D converters. Those can easily do 20 bits, with many available better than that. Measuring the input signal and converting it to digital is therefore no problem.
The digital signal can be transmitted across isolation easily. In this case, a single opto-coupler would do. You don't even need a fast one. Let's say you use UART-style serial data at 300 baud, and you transmit 3 bytes per sample. That allows transmitting 10 samples/second, which is enough to carry 1 Hz bandwidth quite comfortably.
Since you have 5 V available on the receiving side, it's no problem to capture the output of the opto-coupler, feed it into a UART, process the bytes from the UART, and drive a D/A accordingly. It would be hard to find a microcontroller that couldn't do this rather easily. All you need is one with a built-in UART, which is most of them. There are plenty of 16 bit D/As out there, so that's no problem either.
You might even look at filtered PWM out of the microcontroller instead of a separate D/A. To get the equivalent of 16 bits out, you need 65535 PWM slices in a PWM period. For 10 samples/s, you need a minimum PWM clock of 655,350 Hz. Finding a micro that can do 1 MHz PWM is trivial, but you need to look more carefully for one that can do 16 bit duty cycle. Generally, you'll get that with a "16 bit" micro. Pretty much any PIC 24 or dsPIC can do this task.
Keep in mind that one limit on output accuracy is how accurate the voltage reference is that the D/A (in whatever form) uses. 16 bits is one part in 65536, which is 0.0015%. That is actually the toughest requirement of this whole problem.