Is timing available and precise enough to use the triangle channel in such a fashion as to play the rising part of the triangle wave, and then immediately cut and re-start the wave? Or is the "position" of the wave not predictable enough to allow for this?
I remember seeing a Saw wave demo some time ago and I have been curious how this was achieved.
Saw wave demo was using the DMC channel, some pcm value writes, and interrupts.
But as for using Triangle wave as a saw wave, you'd need to reset the phase half-way through the wave. And with the exact timing necessary to accomplish this, you're better off just using RAW PCM instead.
Edit: rainwarrior says it doesn't work...
Also, have you seen SuperNSF? Multichannel mod player for the NES!
The triangle cannot be reset.
Blargg did it with DPCM and IRQ.
http://www.slack.net/~ant/misc/nes-saw/
I'm pretty sure that Memblers's Squeedo synth cart utilizes a saw wave on the triangle channel...
Also this thread may be more useful in "NES Music".