The reason that many employees don't do it is not because they are ethical people. It is because they don't know about options and how short works and they don't care and just wait till the options/stocks gets vested and most of the time they do the wise thing and get more money. Even if want to short stocks, they still need to deposit cash into their account to short and if they get laid off before the stock gets vested, they will end up just being short and if the company pays dividend, they have to pay dividend. I lost money in some incidents when I shorted the stock of my company so I had a proof that I don't have insider info otherwise I should have made money.
Definitely you cannot use unvested options or stocks as a collateral for your short position.