Yes, you can use the remaining timer as an 8-bit PWM DAC.
Set it up for 62.5KHz pwm.
Then use phaseaccumulators for creating and adding sawoscillators.
Basically you need 16-bit phaccs with 16-bit increments and add the MSB of them to create the sample.
The adding and PCM output needs to go into you horizontal blanking in the video ISR.
That should give you about 3 saw oscillators.