The "power injector" is called a USB hub that has an external power source. You cannot simply inject power since USB devices will ask for how much they think they need worst case (e.g. iPods that charge via USB will ask for 500ma even if they only need that much to charge the battery, older ones that charge via Firewire ask for 100mA or less so work directly connected). The USB software won't enumerate a device if the power requirements are exceeded.

Using a powered hub, the hub will take a trivial amount of current from the tablet, and any current needs downstream will be satisfied by the hub, not the tablet.