While waiting for parts to arrive I've decided to work a bit on the software side of things.
Well, not actually the software that will run on the Trinket Pro, but some software to help predict what kind of results I can expect from the actual hardware once I get it all set up.
The software currently generates a sine wave target, creates the PWM signal that will feed into the low pass filter, and displays everything in a neat little window. The "Scale" bar actually adjusts the PWM frequency. Not surprising, but this frequency can't be too close to your sine wave frequency or you won't get good results.
To get good results it seems your PWM frequency should be about 10 times the frequency of the highest frequency sine wave you want to generate.
Other wave forms should be simpler to create, especially digital ones.