I don’t think that there was too much that used all of it(and Apple was less than entirely consistent about the behavior of all pins with all devices over the life of the connector, so some of the pins were, if you cared about broad compatibility, something you had to treat as reserved or deprecated at various times); but, aside from low speed, general purpose, Apple Accessory Protocol serial chatter which could be a wide variety of different things to different people, the 30 pin connector had very limited pin re-use/reassignment; so if any accessories used it it is probably in the pinout; even if there isn’t a case where an accessory used two distinct functions at the same time.
If they had wanted to go in this direction I imagine that the ability to act as a USB master could have been exploited for maximum versatility per pin; but I don’t remember Apple being too interested: there was the digital camera focused adapter; but little broader exploitation of ‘generic high speed protocol’ until lightening; which tossed everything but that and power.
