I speculated previously that they might have large, interchangeable plastic plug-in pieces for his back (one with the leg posts, one without) that utilize the spider logo to camouflage any seams when not using the legs.
Pondering this further my previous idea seems unlikely, as that would probably severely limit upper body articulation and/or ab crunch. Flexibility is a key Spidey attribute, so hopefully HT has a better idea.