3.5mm type custom USB connector
Before going off to devise a special connectoring system upon which to deploy USB you should think long and hard why that seems like such a good idea to you. I suspect that you will find feedback from most others that this is not a good idea and that you should stick with standardized connectors that have been already proven for reliable USB operation. 

I can see that you are already wondering about whether this 3.5mm jack idea is going to be a problem or not. I suggest that it will be a problem because you not get the normal GND/VCC make/break behavior that you get for standard USB connectors. Additionally the idea of putting a USB power lead on a 3.5mm jack just sounds like a big short waiting to happen.

Sometimes there are good reasons to use a different connector for USB. One such example is Apple's new Lightning connector used on the iPad Mini and iPad 4. That solution is well thought out and well engineered. And it was done with some very good ideas in mind. One such being that the cable ends of the interface can be inserted either direction making usage much easier for the customers.