That wouldn't work for me, if the value drops below then I would give back the BTC instead as I don't want my NXT or BLK sold or else I would just do that to get the BTC needed.
The BTC isn't needed, it will just bring my profitability up, so I can always return it within seconds.
If the value of the NXT were to fall then I could allow you to repay the loan within a very short timeframe however the threshold would need to be above 110%.
If you can return the BTC within 'seconds' (or hours) then I don't think it would be a problem.
I would say that in the event the value were to drop down to 113% then I could give you a previously agreed upon amount of time to return the BTC
I also would not be holding it on an exchange so any sale would not be 'automatic' or triggered by the exchange