Do you really get enough USD cashback to buy a flight?
Well you would be refunded into Paypal so you have the same problem of getting the money out.
The hotel will know nothing about cashback websites. If you book direct it makes no difference to the hotel what cookies were set in your browser.
But the hotel will know I've booked in US Dollars? Which would be incredibly odd if they know me very well and I'm a U.K citizen with no travels/family in the U.S. I also need the bill to be under a certain amount of GBP Sterling for work purposes, and currency fluctuations could tip it over that amount. It all seems reasonably feasible but just multiple points of inconvenience. If it was 15% HIX U.S (i.e double) then I'd be tempted, otherwise not worth the aggro for me personally.