You would have to pay back, you know that right? You have 20.000 ltc but you have to give those 50 btc back, ltc would have to go drastically up as you said but its impossible to predict that. Anyways 99% of loans here are for values worth < 1 BTC so they are obviously not for that
Why 'drastically'? Any up movement will give you profit (depends on the loan cost and tx fees). And why do you think you can't do this with <1 BTC loan?