You wouldn't want to use an analog octave pedal for long notes. When the note level falls below a certain point, the octave will stop suddenly and unexpectedly. Digital octaves like the Pog and Hog don't have this problem, but don't sound the same as an analog octave.
Instead of octave, maybe try a fuzz which will give you more sustain and harmonic detail. I do 'swells' with a fuzz, low pass filter and an expression pedal - more synth-like than a volume pedal.