I haven't yet had time to figure out how it works, but Ess does have methods for shifting pitch, rate, and tempo. I think they'll be able to solve my particular application.

To create time-stretch you must either play a sound at non-nominal speed and compensate with a pitch shifting transformation (using FFT/IFFT for instance) or play the sound with a granular engine where the playback speed of grains is not correlated to displacement speed of playback head within the sound. The latter means you need a way to play a part of a sound sample from a specific position, not the whole sample from start to end. I don't know if it's possible with Minim currently.