Here's a datasheet for the MSM5232 chip.
http://www.citylan.it/wiki/images/3/3e/5232.pdfFound it through a very roundabout route: first I searched for MSM5232 +arcade, stumbled on some code for the MAME emulator, saw that the chip sometimes had the RS suffix to it, searched for MSM5232RS +pdf +datasheet - voilą !
To add to what GordonJP was saying, I'm afraid it's Squares all the way down. It's a subset of Walsh functions called Rademacher functions. There are some Walsh harmonics missing that you could re-create by using the existing Square harmonics and EXORing them but... see below.
So, there's no real Saw output at all, the only thing you'll get is that the harmonics have a special amplitude configuration so that a mixture of those squares approximates a Saw...
Theoretically, Walsh synthesis is as versatile as Fourier synthesis (which does a similar thing with Sines), but for more flexibility, you'd need a good set of harmonics (i.e. 8 or 16 of them).
Once you have these, of course, you'd also need to be able to manipulate each harmonic's amplitude in a flexible way, i.e. either manually or automatically, and find some way of storing these settings.
So, it becomes a problem of control.
In truth, instead of going this route with the Poly-800, it's probably much simpler to mix in your DIY OSC circuit into the filter to augment the Poly-800's limited Osc section. You could tap into the MIDI OUT and route that to your new OSC(s) to get tracking.