Eliminating oscillation in a feedback circuit
it's effectively a comparator relative to 1.25V. 


It's not a comparator, it's an error amplifier ("EA" on the block diagram). The 33nF cap and the internal series RC are part of the frequency compensation. 

It's unclear what kind of voltage range you're trying to achieve. Maybe you want something like Vfb = +1.25V + a*Vout- Vpwm, which would ideally yield an output voltage of Vpwm/a where a &lt; 1. 

You could accomplish that with a non-critical op-amp to buffer the filtered PWM and a relatively high bandwidth differential amplifier (two op-amps total). And maybe a reference such as a TLV431 or MCP1501.  

If your op-amp has sufficiently high bandwidth compared to the amplifier in the IC, then it might be sufficient stable without adding further compensation components.