Working out some details, I noticed that the PCF8591T only works at the 100kHz speed on the I2C bus, and I'm not sure how well audio will transmit both ways well through the bus at that speed (especially since the device I'll mainly be using this for also uses an EchoWing, which also drives audio along the I2C bus both directions, making four signals along that bus). I took a look around for something that could go on the 400kHz speed, and found the Analog Devices AD5593R will do that.
The bad news is that the AD5593R has many bells and whistles (it's an 8-channel configurable chip, with each channel configurable to ADC, DAC, or GPIO) that I won't use, but that drive the price up another $2-3. Given that I'm trying to keep the price down, I think I'll hold off on that configuration, unless someone advises otherwise.
(I did put together the layout, though, so it'll be ready just in case.)