Please post a sketch if you try this. I would be very interested to see this done on Bela.
Another possible approach may be based on Stephen Bernsee's pitch shift algorithm:
Time-stretching can be accomplished by pitch shifting and oversampling. Computationally efficient example for slowing down playback would be:
Pitch shift up
Resample with linear interpolation to fill in more samples for the slower playback rate.
I don't know if this is any more low CPU than rubber band library but something worth trying if you find rubber band doesn't work on Bela.
Using larger FFT block sizes and lower sample rates I have successfully run the Stephen Bernsee on CPU's similar to BBB, so I at least have some hint that it could work in real time on Bela.