I am sure they are out there. I ordered this a few days ago so I could charge and use HDMI out: http://www.ebay.com.au/itm/181285387389 haven't received it yet so no idea how well it works...
Not sure if the USB charging cable has the full USB pin outs or just charging pins. If it has full pinout then it may be possible to connect it to a USB hub and then connect other devices as well.
You can't charge when the tablet is in usb host. On the nexus 7 there is a kernel mod that allow this, because usb supports it, but i guess not many android devices implement it.