I think the "threshold" for risking losing 99.99% of earning 12.5 BTC is a gamble of how many percent of their hashpower is to the total network of about double or triple. Or 25 BTC. Or more.
No miner in his right mind will give up the current block reward for anything less than double or triple that. I think no miner would give up 12.5 BTC unless the gamble has a higher than 50% chance of earning 25 BTC, or a higher than 75% of earning 50 BTC (if the bet or prize gets that big).
Now if the last digit of a block hash is used in conjunction with a secret, then the website owner would have to collude with a large or strong miner or a pool.
Currently, there is no known single pool or sufficiently large solo miner that has the hashpower to even consider this kind of attack, plus it is a gamble, so why would they do it?
So, the winning amount, i.e. 16 BTC, is more than a block reward, i.e. 12.5 BTC. Do u think, miners may hold a block now if they place 1 BTC bet with a wrong choice? Appears impossible to me...
Nope. Too risky for a winning amount that small. I'd rather take the 12.5 BTC because I have that now. (not even guaranteed).