You can find persuasion in varying form everywhere. The minister will persuade you to the church, the politicians want you to join his party or vote for him or a sales man wants to sell his products. All are trying to influence to get some benefit for them.
The boss of your company may want you to support his ideas, co-workers may want you to support them, and friends will need you to accompany them, advertisers will try to sell their products. The reason for persuasion only change but the intention is same. The person you creates more influence wins.
Your child may go after drug dealers. Whom can you blame, the drug dealer, his friends who took him or you for having no influence? So the degree of influence is also different. Persuasion, influence, salesmanship, manipulation all are from the same root, someone trying to make others do something. The techniques used by the persuaders also vary, some will be polished but others will be crude in their approach.
All of us will persuade others to do something for us at some stage of life. So when we think about the ethics of covert hypnosis actually we should evaluate whether persuasion is right or wrong. Covert hypnosis is actually a method of persuasion. So if persuasion is right then covert hypnosis is also right. Persuasion is good if it gives some help or benefit to the subject and it is bad if it is harming the person. So before persuading others to do something ask you whether you are helping him or harming.
You can use any technique to perform covert hypnosis to succeed. None of the tools is immoral and has no harmful effect on the subject. The only thing you have to consider is the fact whether you are doing the right thing to the subject. If you feel you are right and honest, there is no moral or legal obstacle to applying covert hypnosis.