So now I'm on 8.1 Pro... and my cheapy OTG hub is actually providing power and data at the same time... albeit from what I gather is already a much reduced rate. Certainly points to software rather than hardware!
So... using a USB power meter (Worthwhile buy - between £5-£10 off ebay. Can get a basic one, or one with an OLED display like below) I've measured the input into the tablet whilst using the OTG hub and without. This is using the stock USB charger, which is rated for 2 amps.
Power direct to tablet:
Power via OTG hub:
As you can see, there's quite a difference, over 1/3 more power being provided when skipping the OTG hub.