heh, it can be done, but the money spend be enough to buy a cheap bass and play.
Anyway if still interested, roland guitar synth is the answer. Using the guitar and synth mix setting, one is able to blend slapbass/fretted bass/fretless bass/double bass tone with the guitar part. It sounds better than individual octave pedal imho, but still lacking in certain organic feel, compare to a real bass.