What are some ways I can modulate the duty cycle of SPWM with mixed signals?
It's possible you got stuck in the meaning of SPWM (sinusoidal pulse-width modulation), but what you're talking about is just plain PWM: usually there is a ramp, or triangle, and a signal which is compared to the ramp/triangle, generating a variable-width pulse of fixed frequency. There are also fixed-width, variable frequency modulators, and they're just another flavour of PWM.
This question deals with about the same principle. It doesn't matter how your signal looks as long as its bandwidth is less than twice the carrier's frequency (for a proper Nyquist sampling).