Tools for simulating software-driven PWM
Sorry, but it is not very clear what are your requirements.

Anyway, Multisim, which is available for free from National Instruments with some sort of student license, is also a SPICE simulator and among its models has a PIC simulation engine, i.e. you can enter PIC assembly code. I never used it myself, but I knew a couple of people that said it is decent. YMMV

But you say you don't need to simulate the PIC, so maybe you could use LTspice, it is a free professional SPICE simulator. It supports behavioral voltage sources, i.e. sources whose output is modeled by mathematical functions. Excerpts from its help file:


  B. Arbitrary Behavioral Voltage or Current Sources Symbol names: BV,
  BI
  
  Syntax: Bnnn n001 n002 V= [ic=]
  + [tripdv=] [tripdt=]
  + [laplace= [window=]
  + [nfft=] [mtol=]]


...


  Tripdv and tripdt control step rejection. If the voltage across a
  source changes by more than tripdv volts in tripdt seconds, that
  simulation time step is rejected.
  
  Expressions can contain the following:
  
  o Node voltages, e.g., V(n001)
  
  o Node voltage differences, e.g., V(n001, n002)
  
  o Circuit element currents; for example, I(S1), the current through
  switch S1 or Ib(Q1), the base current of Q1. However, it is assumed
  that the circuit element current is varying quasi-statically, that is,
  there is no instantaneous feedback between the current through the
  referenced device and the behavioral source output. Similarly, any ac
  component of such a device current is assumed to be zero in a small
  signal linear .AC analysis.
  
  o The keyword, "time" meaning the current time in the simulation.
  
  o The keyword, "pi" meaning 3.14159265358979323846.
  
  o The following functions:


...

And then lists a lot of functions and operators. Refer to the guide for more details or this very complete LT WIKI page.

You can even specify the source behavior using Laplace transforms.

Maybe it can be useful for you: you can enter the schematic of your analog part and model the interactions using behavioral sources.